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Printed: April 5, 2012
Douglas Wilkin, Ph.D. (DouglasW)
MS Studying the Life Sciences
Scientific Ways of Thinking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
What Are the Life Sciences? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Scientific Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tools of Science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Safety in Scientific Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MS What is a Living Organism?
Characteristics of Living Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chemicals of Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Classification of Living Things . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MS Cells and Their Structures
Introduction to Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cell Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MS Cell Functions
Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Photosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cellular Respiration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MS Cell Division, Reproduction, and DNA
Cell Division . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reproduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
DNA, RNA, and Protein Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
MS Genetics
Gregor Mendel and the Foundations of Genetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Modern Genetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
Human Genetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
Genetic Advances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
MS Evolution
Evolution by Natural Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
Evidence of Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
Macroevolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
History of Life on Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
MS Prokaryotes
Bacteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
Archaea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
MS Protists and Fungi
Protists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
Fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
10 MS Plants
10.1 Introduction to Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
10.2 Seedless Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
10.3 Seed Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
10.4 Plant Responses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269
11 MS Introduction to Invertebrates
11.1 Overview of Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280
11.2 Sponges and Cnidarians . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285
11.3 Worms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
12 MS Other Invertebrates
12.1 Mollusks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301
12.2 Echinoderms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
12.3 Arthropods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317
12.4 Insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329
13 MS Fishes, Amphibians, and Reptiles
13.1 Introduction to Vertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344
13.2 Fishes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
13.3 Amphibians . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358
13.4 Reptiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365
14 MS Birds and Mammals
14.1 Birds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381
14.2 Mammals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392
14.3 Primates and Humans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
15 MS Behavior of Animals
15.1 Understanding Animal Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413
15.2 Types of Animal Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428
16 MS Skin, Bones, and Muscles
16.1 Organization of Your Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443
16.2 The Integumentary System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453
16.3 The Skeletal System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462
16.4 The Muscular System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473
17 MS Food and the Digestive System
17.1 Food and Nutrients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483
17.2 Choosing Healthy Foods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493
17.3 The Digestive System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502
18 MS Cardiovascular System
18.1 Introduction to the Cardiovascular System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515
18.2 Heart and Blood Vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526
18.3 Blood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 531
18.4 Health of the Cardiovascular System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 541
19 MS Respiratory and Excretory Systems
19.1 The Respiratory System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 550
19.2 Health of the Respiratory System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 558
19.3 The Excretory System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 568
20 MS Controlling the Body
20.1 The Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579
20.2 Eyes and Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591
20.3 Other Senses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601
20.4 Health of the Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 610
21 MS Diseases and the Body’s Defenses
21.1 Infectious Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 621
21.2 Noninfectious Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 629
21.3 First Two Lines of Defense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 638
21.4 Immune System Defenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 644
22 MS Reproductive Systems and Life Stages
22.1 Male Reproductive System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655
22.2 Female Reproductive System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 660
22.3 Reproduction and Life Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 666
22.4 Reproductive System Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 677
23 MS From Populations to the Biosphere
23.1 Introduction to Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687
23.2 Populations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 692
23.3 Communities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 700
23.4 Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707
23.5 Biomes and the Biosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713
24 MS Ecosystem Dynamics
24.1 Flow of Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723
24.2 Cycles of Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 732
24.3 Ecosystem Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739
25 MS Environmental Problems
25.1 Air Pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 747
25.2 Water Pollution and Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 755
25.3 Natural Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 761
25.4 Habitat Destruction and Extinction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 771
26 MS Life Science Glossary
26.1 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 786
26.2 B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 789
26.3 C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 791
26.4 D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795
26.5 E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 797
26.6 F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 800
26.7 G . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 802
26.8 H . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804
26.9 I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 806
26.10J . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 808
26.11K . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 809
26.12L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810
26.13M . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 812
26.14N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815
26.15O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817
26.16P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 818
26.17Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822
26.18R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823
26.19S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825
26.20T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 829
26.21U . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 831
26.22V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 832
26.23W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 833
26.24X . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834
26.25Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 835
26.26Z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 836
1 MS Studying the Life Sciences
How many questions can you ask about the sun? Why does the sun shine? How does it move across the sky? How
does it help plants grow? What would happen to life on Earth if the sun failed to rise tomorrow? You could ask all of
these questions, and probably many more. When scientists observe the physical world, they ask questions like these
and then try to answer them.
Before science, these types of questions were answered by beliefs and myths. For example, ancient Aztec societies
believed the sun would not rise unless they performed human sacrifices. Science teaches us that we need evidence
that can explain our observations. In this chapter, we will explore how science can help us ask and answer the
questions we have about our physical world.
Scientific Ways of Thinking
Lesson Objectives
• Describe the role of a scientist.
• Understand that science is a system based on evidence, testing, and reasoning.
evidence Something that gives us grounds for knowing of the existence or presence of something else.
experiment A test to see if a hypothesis is right or wrong; a test to obtain new data.
Modern Science
Modern science is:
a. A way of understanding the physical world, based on observable evidence, reasoning, and repeated testing.
b. A body of knowledge that is based on observable evidence, experimentation, reasoning, and repeated testing.
Thinking Like a Scientist
How can you think like a scientist?
• Scientists ask questions: The key to being a great scientist is to ask questions. Imagine you are a scientist
in the African Congo. While in the field, you observe one group of healthy chimpanzees on the North side
of the jungle. On the other side of the jungle, you find a group of chimpanzees that are mysteriously dying.
What questions might you ask? A good scientist will ask, "What differs between the two environments where
the chimpanzees live?" and "Are there differences in behavior between the two chimps that allow one group
to survive over another?"
• Scientists make detailed observations: A person untrained in the sciences may observe, "The chimps on
one side of the jungle are dying, while chimps on the other side of the jungle are healthy." Can you think of
ways to make this observation more detailed? What about the number of chimps? Are they male of female?
Young or old? A good scientist may observe, "While all seven females and three males on the North side
of the jungle are healthy and show normal behavior, four female and five male chimps under the age of five
have died." Detailed observations can ultimately help scientists to design their experiments and answer their
questions. See a photo of chimpanzees in Figure 1.1 .
An adult and child chimpanzee.
• Scientists find answers using tests: When scientists want to answer a question, they search for evidence
using experiments. An experiment is a test to see if a hypothesis is right or wrong. Evidence is made up
of the observations a scientist makes during an experiment. To study the cause of death in the chimpanzees,
scientists may give the chimps nutrients in the form of nuts, berries, and vitamins to see if they are dying from
a lack of food. This test is the experiment. If fewer chimps die, then the experiment shows that the chimps
may have died from not having enough food. This is the evidence.
• Scientists question the answers: Good scientists are skeptical. Scientists never use only one piece of evidence to form a conclusion. For example, the chimpanzees in the experiment may have died from a lack of
food, but can you think of another explanation for their death? They may have died from a virus, or from
another less obvious cause. More experiments need to be completed before scientists can be sure. Good scientists constantly question their own conclusions. They also find other scientists to confirm or disagree with
their evidence.
What Are the Life Sciences?
Lesson Objectives
• Define Life Science.
• Describe how evidence is used to create and support scientific theories.
cell theory All organisms are composed of cells; cells are the basic units of structure and function in an organism;
cells only come from preexisting cells.
life science The study of living organisms, and how they interact with each other and their environment.
scientific theory A well-established set of explanations that explain a large amount of scientific information.
theory of evolution Theory developed by Charles Darwin that explains how populations of organisms can change
over time.
Fields in the Life Sciences
The life sciences are the study of living organisms, and how they interact with each other and their environment.
Life sciences deal with every aspect of living organisms.
The life sciences are so complex that most scientists focus on just one or two subspecialties. If you want to study
insects, what would you be called? An entomologist. If you want to study the tiny things that give us the flu, then
you need to enter the field of virology. Look at Table 1.1 , Table 1.2 , and Table 1.3 . If you want to study the
nervous system, what life sciences field is right for you?
TABLE 1.1:
Marine biology
organisms living in and
around oceans, and seas
Fresh water biology
viruses ( see Figure 1.2 )
the classification of organisms
organisms living in and
around freshwater lakes,
streams, rivers, ponds, etc.
TABLE 1.2: Fields of life sciences that examine the structure, function, growth, development
and/or evolution of living things
Life Science
Cell biology
Developmental biology
and embryology
What it Examines
cells and their structures
(see Figure 1.2 )
the form and structure of
living organisms
Life Science
What it Examines
the structures of animals
the mechanisms inside organisms that protect them
from disease and infection
the growth and development of plants and animals
the chemistry of living organisms
how diseases arise and
the physical and chemical
functions of tissues and
the nervous system
Molecular biology
the genetic make up of all
living organisms (heredity)
biology at the molecular
TABLE 1.3: Fields of biology that examine the distribution and interactions between organisms
and their environments
Life Science
Population biology
What it Examines
how various organisms interact with their environments
the biodiversity, evolution, and environmental
biology of populations of
Life Science
What it Examines
the distribution of living
organisms (see Figure 1.3
Scientific Evidence and Theories in the Life Sciences
Scientists perform experiments and collect evidence. Evidence is:
a. A direct, physical observation of a thing, a group of things, or a process over time.
b. Usually something measurable or "quantifiable."
c. The data resulting from an experiment.
For example, an apple falling to the ground is evidence in support of the theory of gravity (Figure 1.4 ). A bear
skeleton in the woods would be evidence of the presence of bears.
Scientific theories are well established and tested explanations of observations or evidence. Scientific theories are
produced through repeated experiments. Theories are usually tested and confirmed by many different people.
Scientific theories produce information that helps us understand our world. For example, the idea that matter is made
up of atoms is a scientific theory. Scientists accept this theory as a fundamental principle of basic science. However,
when scientists find new evidence, they can change their theories.
This drawing shows red blood cells and
viruses. Virology is the study of viruses.
Cell biology is the study of cells.
Biogeography looks at the variation of
life forms within a given ecosystem
biome or for the entire Earth. In other
words it tries to explain where organisms live and at what abundance.
An apple falls from a tree to the ground
instead of floating in space. This is evidence for the theory of gravity.
The following are links to information about the nature of science:
Two Important Life Science Theories
In the many life sciences, there are possibly hundreds or thousands of theories. The field of modern biology, however,
really depends on two especially significant theories. They are:
a. The Cell Theory
b. The Theory of Evolution.
The Cell Theory
The Cell Theory states that:
a. All organisms are composed of cells (Figure 1.5 ).
b. Cells are the basic units of structure and function in an organism.
c. Cells only come from preexisting cells; life comes from life.
The development of the microscope in the mid 1600s made it possible for scientists to develop this theory (Figure
1.6 ).
The two types of cells eukaryotic le f t
and prokaryotic right .
A mouse cell viewed through a microscope.
The Theory of Evolution
The Theory of Evolution explains how populations of organisms can change over time. It also explains why there
are many different types of organisms on Earth. This theory is often called the "great unifier" of biology, because it
applies to every field of biology. It also explains how all living organisms on Earth come from common ancestors
(Figure 1.7 ). You will learn more about the details of the theory of evolution in later chapters.
Evolution explains the millions of varieties of organisms on Earth.
An introduction to evolution and natural selection can be viewed at
Click image to the left for more content.
Lesson Summary
• Science is a way of understanding about the physical world.
• Science is based on evidence, reasoning, and testing predictions.
• Information that has been thoroughly tested can still undergo further testing and revisions, as new evidence
and questioning are raised.
• Science differs from other ways of knowing because it is entirely based on observable evidence.
• Scientific explanations are constantly questioned and tested.
• Science produces theories and general knowledge.
• Science allows us to better understand the world.
• Science allows us to apply this knowledge to solve problems.
Review Questions
1. What do all fields of life science have in common?
2. What are the three characteristics of evidence?
3. What is the goal of science?
Apply Concepts
4. What would you study if you were a biogeographer?
5. Why do you think the development of microscopes led to the development of the Cell Theory?
Think Critically
6. What do you think the difference is between a theory and a set of observations?
7. If all cells come from pre-existing cells, where do you think the first cells came from?
Further Reading / Supplemental Links
• Moore, John, A., Science as a Way of Knowing: The Foundations of Modern Biology. Harvard University
Press, 1993.
• Trefil, James, The Nature of Science, An A-Z Guide to the Laws and Principles Guiding the Universe.
Houghton Mifflin, Boston, 2003.
• Darwin, Charles, Origin of the Species. Random House 1988.
• Cromer, Alan, Uncommon Sense: The Heretical Nature of Science, Oxford University Press. 1993.
• The Nature of Science the Prentice Hall Science Series 1993.
• American Association for the Advancement of Science. Science for All Americans. 1993.
• Johnson, Rebecca, Genetics (Great Ideas of Science), Twenty-First Century Books, 2006.
• Hedrick, Philip, W., Genetics of Populations (Biological Science (Jones and Bartlett)) Jones and Brothers
Publishers, 2005.
• Charles Darwin: And the Evolution Revolution (Oxford Portraits in Science) by Rebecca Stefoff. Oxford
Press. New York, 1996.
• Fleisher, Paul, Evolution (Great Ideas of Science). TwentyFirst Century Books, 2006..
Points to Consider
Next we discuss the scientific method.
• How does thinking like a scientist allow us to answer questions about life?
• What is the difference between science completed in a laboratory and science completed outside?
The Scientific Method
Lesson Objectives
• Describe the scientific method as a process.
• Explain why the scientific method allows scientists and others to examine the physical world more objectively
than other ways of knowing.
• Describe the steps involved in the scientific method.
applied science The application of science to practical problems.
basic science Research whose goal is just to find out how the world works, not to solve an urgent problem. Basic
research is the source of most new scientific information and nearly all new theories.
hypothesis A proposed explanation for something that is testable.
scientific method A careful way of asking and answering questions to learn about the physical world that is based
on reason and observable evidence.
The Scientific Method
The scientific method is a process used to investigate the unknown. This process uses evidence and testing. Scientists use the scientific method so they can find information. A common method allows all scientists to answer
questions in a similar way. Scientists who use this method can reproduce another scientist’s experiments. Why do
you think it is important that scientists reproduce each other’s experiments?
Almost all versions of the scientific method include the following steps, though not always in the same order:
Make observations
Identify a question you would like to answer based on the observation
Find out what is already known about your observation (research)
Form a hypothesis
Test the hypothesis
Analyze your results
Communicate your results
Making Observations
Imagine that you are scientist. While collecting water samples at a local pond, you notice a frog with five legs instead
of four (Figure 1.8 ). As you start to look around, you discover that many of the frogs have extra limbs, extra eyes
or no eyes. One frog even has limbs coming out of its mouth. These are your observations, or things you notice
about an environment using your five senses.
A frog with an extra leg.
Identify a Question Based on Your Observations
The next step is to ask a question about the frogs. You may ask, "Why are so many frogs are deformed?" Or, "Is
there something in their environment causing these defects, like water pollution?"
Yet, you do not know if this large number of deformities is "normal" for frogs. What if many of the frogs found in
ponds and lakes all over the world have similar deformities? Before you look for causes, you need to find out if the
number and kind of deformities is unusual. So besides finding out why the frogs are deformed, you should also ask:
"Is the percentage of deformed frogs in this pond greater than the percentage of deformed frogs in other places?"
(Figure 1.9 )
Research Existing Knowledge About the Topic
No matter what you observe, you need to find out what is already known about your questions. For example, is
anyone else doing research on deformed frogs? If yes, what did they find out? Do you think that you should repeat
their research to see if it can be duplicated? During your research, you might learn something that convinces you to
change or refine your question.
Construct a Hypothesis
A hypothesis is an educated guess that tries to explain an observation. A good hypothesis allows you to make more
predictions. For example, you might hypothesize that a pesticide from a nearby farm is running into the pond and
causing frogs to have extra legs. If that’s true, then you can predict that the water in a pond of non-deformed frogs
will have lower levels of that pesticide.
A pond with frogs.
That’s a prediction you can test by measuring pesticide levels in two sets of ponds, those with deformed frogs and
those with nothing but healthy frogs. Every hypothesis needs to be written in a way that it can:
Be tested using evidence.
Be proven wrong.
Provide measurable results.
Provide yes or no answers.
For example, do you think the following hypothesis meets the four criteria above? Let’s see. Hypothesis: "The
number of deformed frogs in five ponds that are polluted with chemical X is higher than the number of deformed
frogs in five ponds without chemical X."
Test Your Hypothesis
To test the hypothesis, you would count the healthy and deformed frogs and measure the amount of chemical X in all
of the ponds. The hypothesis will be either true or false. Here is an example of a hypothesis that is not testable: "The
frogs are deformed because someone cast a magic spell on them." You cannot test a magic hypothesis or measure
any results of magic. Doing an experiment will test most hypotheses. The experiment may generate evidence in
support of the hypothesis. The experiment may also generate evidence proving the hypothesis false.
Analyze Data and Draw a Conclusion
If a hypothesis and experiment are well-designed, the experiment will produce results that you can measure, collect,
and analyze. The analysis should tell you if the hypothesis is true or false. See Table 1.4 for the experimental
TABLE 1.4: Deformed Frog Data
Polluted Pond
Number of Deformed
Non-polluted Pond
Number of Deformed
|+Deformed Frog Data
Your results show that pesticide levels in the two sets of ponds are different, but the average number of deformed
frogs is almost the same. Your results demonstrate that your hypothesis is false. The situation may be more complicated than you thought. This gives you new information that will help you decide what to do next. Even if the results
supported your hypothesis, you would probably ask a new question to try to better understand what is happening to
the frogs and why.
Drawing Conclusions and Communicating Results
If a hypothesis and experiment are well-designed, the results will tell whether your hypothesis is true or false. If a
hypothesis is true, scientists will often continuing testing the hypothesis in new ways to learn more. If a hypothesis
is false, the results may be used to come up with and test a new hypothesis.
Scientists communicate their results in a number of ways. For example, they may talk to small groups of scientists
and give talks at large scientific meetings. They will write articles for scientific journals. Their findings may also be
communicated to journalists.
If you conclude that frogs are deformed due to a pesticide not previously measured, you would publish an article
and give talks about your research. Your conclusion could eventually help find solutions to this problem.
Basic and Applied Science
Science can be "basic" or "applied." The goal of basic science is to understand how things work - whether it is a cell
or a whole ecosystem. Basic science is the source of most scientific theories and new knowledge. For example, a
scientist that tries to find the right drug to treat brain injuries is performing basic science.
Applied science is using scientific discoveries to solve practical problems. Applied science also creates new technologies. For example, medicine and all that is known about how to treat patients is applied science based on basic
research (Figure 1.10 ). A doctor administering a drug or performing surgery on a patient is an example of applied
Surgeons operating on a person an example of applied science.
Lesson Summary
The scientific method is a process used to investigate questions.
The scientific method uses observable evidence and testing.
A hypothesis is a proposed explanation of an observation; it is used to test an idea.
A hypothesis must be written in a way that can be tested, can be proved false, can be measured, and will help
answer the original question.
• Basic research produces knowledge and theories.
• Applied research uses knowledge and theories from basic research to develop solutions to practical problems.
Review Questions
1. What does a hypothesis need to include?
2. What does "falsifiable" mean?
3. List the steps of the Scientific Method.
Apply Concepts
4. How is a hypothesis different from a theory?
5. A doctor treats a patient with HIV with a new anti-viral drug. Is this an example of basic or applied science?
Think Critically
6. What does a scientist do if their research contradicts previous theories or popular knowledge?
7. A field scientist studies mice and observes that mice in the desert have fewer offspring (children) than mice in
the forest. She hypothesizes that mice in the desert have access to less water and therefore have fewer offspring to
conserve the much-needed resource. Is this a testable hypothesis? Why or why not?
Further Reading / Supplemental Links
• William Souder, A Plague of Frogs: The Horrifying True Story. Hyperion Press, 2000.
Points to Consider
• How do you think scientific “tools” can help scientists?
• What do you think is one of the more common tools of the life scientist?
Tools of Science
Lesson Objectives
• Describe the growing number of tools available to investigate different features of the physical world.
• Describe how microscopes have allowed humans to view increasingly small tissues and organisms that were
never visible before.
electron microscope Microscope used to create high magnification (magnified many times) and high resolution
(very clear) images.
microscope A set of lenses used to look at things too small to be seen by the unaided eye.
microscopy All the methods for studying things using microscopes.
optical (light) microscope A microscope that focuses light, usually through a glass lens; used by biologists to
visualize small details of biological specimens.
scanning acoustic microscope A microscope that focuses sound waves instead of light.
scanning electron microscope (SEM) A microscope that scans the surfaces of objects with a beam of electrons
to produce detailed images of the surfaces of tiny things.
transmission electron microscope (TEM) A microscope that focuses a beam of electrons through an object and
can make an image up to two million times bigger, with a very clear image ("high resolution").
Using Microscopes
Microscopes, tools that you may get to use in your class, are some of the most important tools in biology (Figure
1.11 ). Look at your fingertips. Before microscopes were invented in 1595, the smallest things you could see on
yourself were the tiny lines in your skin. But what else is hidden in your skin?
Over four hundred years ago, two Dutch spectacle makers, Zaccharias Janssen and his son Hans, were experimenting
with several lenses in a tube. They discovered that nearby objects appeared greatly enlarged. That was the forerunner
of the compound microscope and of the telescope. Later, the father of microscopy, Dutch scientist Antoine van
Leeuwenhoek (Figure 1.12 ) taught himself to make one of the first microscopes (Figure 1.13 ). A microscope is
a tool used to make things that are too small to be seen by the human eye look bigger. Microscopy is a technology
for studying small objects using microscopes.
In 1665, Robert Hooke, an English natural scientist, used a microscope to zoom in on a piece of cork — the stuff
that makes up the stoppers in wine bottles. Inside of cork, he discovered the smallest building blocks of life, or cells
(Figure 1.14 and Figure 1.15 ). This finding eventually led to the development of the theory that all living things
are made up of cells. Without microscopes, this theory would not have been developed.
In one of his early experiments, van Leeuwenhoek took a sample of scum from his own teeth and used his microscope
to discover bacteria, one of the tiniest living organisms on the planet. Using microscopes, van Leeuwenhoek also
discovered one-celled organisms (protists) and sperm.
Basic light microscopes opened up a
new world to curious people. See if you
can identify the following parts of the
microscope 1 ocular lens or eyepiece
2 objective turret 3 objective lenses 4
coarse adjustment knob 5 fine adjustment knob 6 object holder or stage 7
mirror or light illuminator 8 diaphragm
and condenser.
Some modern microscopes use light, as Hooke’s and van Leeuwenhoek’s did, but others may use electron beams or
sound waves.
Researchers now use these types of microscopes:
a. Light microscopes allow biologists to see small details of a specimen. Most of the microscopes used in
schools and laboratories are light microscopes. Light microscopes use refractive lenses, typically made of
glass or plastic, to focus light either into the eye, a camera, or some other light detector. The most powerful
light microscopes can make images up to 2,000 times larger.
b. Transmission electron microscopes (TEM) focus a beam of electrons through an object and can make an
image up to two million times bigger, with a very clear image ("high resolution").
c. Scanning electron microscopes (SEM) (Figure 1.16 and Figure 1.17 ) allow scientists to find the shape
and surface texture of extremely small objects, including a paperclip, a bedbug, or even an atom. These
microscopes slide a beam of electrons across the surface of a specimen, producing detailed maps of the shapes
of objects. Visit the following site to see incredible images of tiny organisms and objects using an electron
scanning microscope:
d. Scanning acoustic microscopes use sound waves to scan a specimen. These microscopes are useful in biology
and medical research.
Other Life Science Tools
What other kinds of tools and instruments would you expect to find in a biologist’s laboratory or field station? Other
than computers and lab notebooks, biologists use very different instruments and tools for the wide range of life
Antoine van Leeuwenhoek a Dutch
cloth merchant with a passion for microscopy.
Drawing of microscopes owned by Antoine van Leeuwenhoek. Bacteria were
discovered in 1683 when Antoine Van
Leeuwenhoek used a microscope he
built to look at the plaque on his own
Robert Hookeś early microscope.
Cell structure of cork by Hooke.
A scanning electron microscope.
A scanning electron microscope image
of blood cells.
science specialties. For example, a medical research laboratory and a marine biology field station might not use any
of the same tools.
Tools such as a radiotelemetry device (Figure 1.18 ), a thermocycler (Figure 1.19 ) and inoculation (sterile) hoods
(Figure 1.20 ) are all biological equipment.
A radiotelemetry device used to track
the movement of seals in the wild.
A thermocycler used for molecular biological and genetic studies.
A sterile laboratory chamber. This laboratory inoculation hood allows researchers to conduct experiments in
sterile environments.
Using Maps and Other Models
People use models for many purposes. We use street maps to help us find our way around. A model of the solar
system may show the relationship between the planets in space. Life scientists often use maps to show where different organisms live, or to learn about the climate of an area. Scientists use maps and models to explain observations
and to make predictions. For example, if a scientist wants to study the effect of temperature on coral reefs, he or she
would first consult a map of coral reef locations and then measure the temperature in those specific areas (Figure
1.21 ).
The above map shows where you can find coral reefs around the world. Coral reefs are shown in red.
Some models are used to show the relationship between different parts of an experiment, or variables. For example,
the model in Figure 1.22 says that when there are few coyotes, there are many rabbits (left side of the graph) and
when there are only a few rabbits, there are many coyotes (right side of the graph). You could make a prediction,
based on this model, that removing all the coyotes from the ecosystem would result in an increase in rabbits. This is
a good scientific prediction because it can be tested.
This graph shows a model of a relationship between a population of coyotes
the predators and a population of rabbits which the coyotes eat the prey.
Lesson Summary
• From the time that the first microscope was built, over four hundred years ago, microscopes have been used to
make major discoveries.
• Life science is a vast field; different kinds of research usually require very different tools.
• Scientists use maps and models to understand how features of real events or processes work.
Review Questions
1. What did van Leeuwenhoek discover when he looked at scum from his own teeth under the microscope?
2. What does the symbol 10X on the side of a microscope mean?
3. What is a scientific model?
Apply Concepts
4. Look at the predator/prey (coyote/rabbit) model in Figure 1.22 . What does the model predict would happen to
the rabbit population if you took away all of the coyotes?
Think Critically
5. If you want to describe all of the places on the planet where ants can survive, how would you display this
6. What tool might you use to keep track of where a wolf travels?
Points to Consider
• What hazards may biologists face in the laboratory?
• What could be risks may biologists face who complete research outside?
• What do you think biologists do to protect themselves?
Safety in Scientific Research
Lesson Objectives
• Recognize how the kind of hazards that a scientist faces depends on the kind of research they do.
• Identify some potential risks associated with scientific research.
• Identify how safety regulations protect scientists and the environment.
biohazard Any biological material, such as infectious material that poses a potential to human health, animal
health, or the environment.
carcinogen Anything that can cause cancer.
field scientist Scientists who work outdoors.
teratogen A chemical that causes deformities.
Types of Safety Hazards
There are some very serious safety risks in scientific research. When studying a science like chemistry, scientists
need to make sure that they do not mix two explosive chemicals together. Since the life sciences deal with living
organisms, some research may have risks not found in other fields. Safety practices must be followed when working
with the following hazardous things:
Disease-causing viruses, bacteria or fungi
Wild animals
Radioactive materials
Pollutants in air, water, or soil
The kinds of risks that scientists face depend on the kind of research they perform. For example, a bacteriologist
working with bacteria in a laboratory faces different risks than a zoologist studying the behavior of lions in Africa.
Think back to the deformed frogs discussed earlier. If there is something in the frogs’ environment causing these
deformities, could there be a risk to a researcher in that environment? A chemical in the pond that could cause
such deformities is called a teratogen. If the chemical is causing deformities in frogs, could it cause deformities in
humans? A scientist would most likely wear gloves and maybe even a mask when dealing with polluted water.
Or perhaps a disease is causing the deformities. Viruses and bacteria are called biohazards. Figure 1.23 shows
laboratory safety and chemical hazard signs. Biohazards include any biological material that could make someone
sick. A used needle or laboratory bacteria are also biohazards.
Science Laboratory Safety and Chemical Hazard Signs.
Laboratory Safety
If you perform an experiment in your classroom, your teacher will explain how to be safe. Professional scientists
follow safety rules as well, especially for the study of dangerous organisms like the bacteria that cause bubonic
plague, as shown in Figure ?? .
Sharp objects, chemicals, heat, and electricity are all used at times in laboratories. Below is a list of safety guidelines
that you should follow when doing labs:
Be sure to obey all safety guidelines given in lab instructions and your teacher.
Follow directions carefully.
Tie back long hair.
Wear closed shoes with flat heels and shirts with no hanging sleeves, hoods, or drawstrings.
Use gloves, goggles, or safety aprons when instructed to do so.
Broken glass should only be cleaned up with a dust pan and broom. Never touch broken glass with your bare
• Never eat or drink anything in the science lab. Table tops and counters could have dangerous substances on
• Be sure to completely clean materials like test tubes and beakers. Leftover substances could interact with other
substances in future experiments.
• If you are using flames or heat plates, be careful when you reach. Be sure your arms and hair are kept far away
from heat.
• Alert your teacher immediately if anything out of the ordinary occurs. An accident report may be required if
someone is hurt and the lab supervisor must know if any materials are damaged or discarded.
Field Research Safety
Scientists who work outdoors, called field scientists, are also required to follow safety regulations designed to
prevent harm to themselves, other humans, to animals, and the environment. In fact, if scientists work outside the
country, they are required to learn about and follow the laws and restrictions of the country in which they are doing
research. For example, entomologists following monarch butterfly (Figure 1.24 ) migrations between the United
States and Mexico must follow regulations in both countries.
A Monarch Butterfly
Field scientists are also required to follow laws to protect the environment. Before biologists can study protected
wildlife or plant species, they must apply for permission to do so, and obtain a research permit. For example, if
scientists collect butterflies without a permit, they may unknowingly disturb the balance of the organism’s habitat.
Lesson Summary
• Research of any kind may have safety risks. Because life scientists study organisms as diverse as bacteria and
bears, they deal with risks that other scientists may never encounter.
• The risks scientists face depend on the kind of research they are doing.
• Scientists are required by federal, state, and local institutions to follow strict regulations designed to protect
the safety of themselves, the public, and the environment.
Review Questions
1. What kinds of hazards might be found in biology laboratories, but not physics laboratories?
2. Who has more freedom to do whatever research they want? Laboratory scientists or field biologists?
3. What is a biohazard?
4. What is a research permit?
Apply Concepts
5. What are some of the precautions you might take if you were collecting frogs in water you think might be
6. Name some possible hazards to field biologists.
Think Critically
7. You want to complete field research on the border between the United States and Mexico. Before you begin, what
precautions should you take?
Further Reading / Supplemental Links
Biosafety in Microbiological and Biomedical Laboratories (National Research Council, 1999).
Chemical Classification Signs: jimaskew/nfpa.htm
NFPA Chemical Hazard Labels:
Where to Find MSDS’s on the Internet:
MSDS Power Point: http://www.research.nort
Points to Consider
• What do you think makes something “alive?”
• What does a blade of grass, a fly, and a human have in common?
MS What is a Living
How do we tell the difference between a living thing and a non-living thing? Think about your own body. How do
you know that you are alive? Your heart beats. You breathe in air. Do all living things need to do be like you in order
to be "alive"?
The above image represents bacteria. Do these bacteria look like they could be alive? They do not have hands or feet
or a heart or a brain, but they are actually more similar to you than you may think. Scientists found that all living
things share certain characteristics. In this chapter, we will discover how to precisely define living things.
Characteristics of Living Organisms
Lesson Objectives
• List the defining characteristics of living things.
• List the needs of all living things.
cell The smallest living unit of life; the smallest unit of structure and function of living organisms.
embryo An animal or plant in its earliest stages of development, before it is born or hatched.
homeostasis Maintaining a stable internal environment despite changes in the environment.
organism A living thing.
Characteristics of Life
How do you define a living thing? What do mushrooms, daisies, cats, and bacteria have in common? All of these
are living things, or organisms. It might seem hard to think of similarities among such different organisms, but
they actually have many things in common. Living things are similar to each other because all living things evolved
from the same common ancestor that lived billions of years ago. See for a powerful
introduction to life.
All living organisms:
Need energy to carry out life processes.
Are composed of one or more cells.
Respond to their environment.
Grow and reproduce.
Maintain a stable internal environment (homeostasis).
Living Things Need Resources and Energy
Why do you eat everyday? To get energy. The work you do each day, from walking to writing and thinking, is fueled
by energy. But you are not the only one. In order to grow and reproduce, all living things need energy. But where
does this energy come from?
The source of energy differs for each type of living thing. In your body, the source of energy is the food you eat.
Here is how animals, plants and fungi obtain their energy:
• All animals must eat plants or other animals in order to obtain energy and building materials.
• Plants don’t eat. Instead, they use energy from the sun to make their "food" through the process of photosynthesis.
• Mushrooms and other fungi obtain energy from other organisms. That’s why you often see fungi growing on
a fallen tree; the rotting tree is their source of energy (Figure 2.1 ).
Since plants harvest energy from the sun and other organisms get their energy from plants, nearly all the energy of
living things initially comes from the sun.
Bracket fungi and lichens on a rotting
log in Cranberry Glades Park near Marlinton West VIrginia. Fungi obtain energy from breaking down dead organisms such as this rotting log.
Living Things Are Made of Cells
If you zoom in very close on the skin on your hand, you will find cells (Figure 2.2 ). Cells are the smallest unit of
living things. Most cells are so small that they are usually visible only through a microscope. Some organisms, like
bacteria, plankton that live in the ocean, or the paramecium shown in Figure 2.3 are made of just one cell. Other
organisms have millions of cells. On the other hand, eggs are some of the biggest cells around. A chicken egg is just
one huge cell.
The Inner Life of the Cell can be viewed at (5:28).
Click image to the left for more content.
All cells share at least some structures. Although the cells of different organisms are built differently, they all
function much the same way. Every cell must get energy from food, be able to grow and reproduce, and respond to
its environment.
Reptilian blood cell showing the characteristic nucleus. A few smaller white
blood cells are visible. This image has
been magnified 1000 times its real size.
This paramecium is a single-celled organism.
Living Things Respond to their Environment
All living things are able to react to something important or interesting in their external environment. For example,
living things respond to changes in light, heat, sound, and chemical and mechanical contact. Organisms have means
for receiving information, such as eyes, ears, and taste buds.
Living Things Grow and Reproduce
All living things reproduce to make the next generation. Organisms that do not reproduce will go extinct. As a result,
there are no species that do not reproduce (Figure ?? ).
Living Things Maintain Stable Internal Conditions
When you are cold, what does your body do to keep warm? You shiver to warm up your body. When you are too
warm, you sweat to release heat. When any living thing gets thrown off balance, its body or cells help them return to
normal. In other words, living things have the ability to keep a stable internal environment. Maintaining a balance
inside the body or cells of organisms is known as homeostasis. Like us, many animals have evolved behaviors that
control their internal temperature. A lizard may stretch out on a sunny rock to increase its internal temperature, and
a bird may fluff its feathers to stay warm (Figure 2.4 ).
A bird fluffs his feathers to stay warm
keep f rom losing energy and to maintain homeostasis.
Lesson Summary
• All living things grow, reproduce, and maintain a stable internal environment.
• All organisms are made of cells.
• All living things need energy and resources to survive.
Review Questions
1. Define the word organism.
2. What are three characteristics of living things?
Apply Concepts
3. What are a few ways organisms can get the energy they require?
4. What is a cell?
Think Critically
5. Think about fire. Can fire be considered a living thing? Why or why not?
Points to Consider
• DNA is considered the “instructions” for the cell. What do you think this means?
• What kinds of chemicals do you think are necessary for life?
• Do you expect that the same chemicals can be in non-living and living things?
Chemicals of Life
Life on a rocky peak in the Waitakere
Lesson Objectives
• Define matter, element, and atom.
• Name the four main classes of organic molecules that are building blocks of life.
atom The simplest and smallest particle of matter that still retains the physical and chemical properties of the
element; the building block of all matter.
atomic number The number of protons in an element.
carbohydrate Nutrient that include sugars, starches, and fiber; give your body energy; class of organic compound.
chemical reaction A process that breaks or forms the bonds between atoms.
compound Any combination of two or more elements.
electron A negatively charged particle in the atom, found outside of the nucleus.
element A substance that cannot break down into a simpler substance with different properties.
enzyme A substance, usually a protein, that speeds up a biochemical reaction.
lipid Class of organic compound that includes fats, oils, waxes and phospholipids; nutrients, such as fats, that are
rich in energy.
macromolecule Very large molecules that make living organisms; includes carbohydrates, lipids, proteins, and
nucleic acids.
matter Anything that takes up space and has mass.
molecule Any combination of two or more atoms.
neutron The non-charged particle of the atom; located in nucleus of the atom.
nucleic acid Class of organic compound that includes DNA and RNA.
organic compound Compounds made up of a carbon backbone and associated with living things.
Periodic Table Table that organizes elements according to their unique characteristics, like atomic number, density, boiling point, and other values.
product The end result of a reaction.
protein Organic compound made up of smaller molecules called amino acids; performs many functions in the cell.
proton The positively charged particle of the atom; located in nucleus of the atom.
reactant The raw ingredients in a chemical reaction.
Check Your Understanding
• What are the main properties of all living things?
• What is homeostasis?
The Elements
If you pull a flower petal from a plant and break it in half, and take that piece and break it in half again, and take the
next piece, and break it half and so on and so on, until you cannot even see the flower anymore — what do you think
you will find? Scientists have broken down matter, or anything that takes up space and has mass, into the smallest
pieces that cannot be broken down anymore. Rocks, animals, flowers, and your body are all made up of matter (see
2.5 ).
Matter is made up mixture of things called elements. Elements are substances that cannot be broken down into
simpler substances. There are more than 100 known elements, and 92 occur naturally around us. The others have
been made only in the laboratory.
Inside of elements, you will find identical atoms. An atom is the simplest and smallest particle of matter that still
has chemical properties of the element. Atoms are the building block of all of the elements that make up the matter
in your body or any other living or non-living thing. Atoms are so small that only the most powerful microscopes
can see them.
Each element has a different type of atom, and is represented with a one or two letter symbol. For example, the
symbol for oxygen is O and the symbol for helium is He.
Atoms themselves are composed of even smaller particles, including positively charged protons, uncharged neutrons, and negatively charged electrons. Protons and neutrons are located in the center of the atom, or the nucleus,
and the electrons move around the nucleus. How many protons an atom has determines what element it is. For
example, Helium (He) always has two protons (Figure 2.6 ), while Sodium (Na) always has 11. All the atoms of
a particular element have the exact same number of protons, and the number of protons is that element’s atomic
An atom of Helium He contains two
positively charged protons red two uncharged neutrons blue and two negatively charged electrons yellow.
The Element Song can be heard at (1:25)
Click image to the left for more content.
The Periodic Table
In 1869, a Russian scientist named Dmitri Mendeleev created the Periodic Table, which is a way of organizing
elements according to their unique characteristics, like atomic number, density, boiling point, and other values (see
Figure 2.7 ). Each element has a one or two letter symbol. For example, H stands for hydrogen and Au for gold.
The vertical columns in the periodic table are known as groups, and elements in groups tend to have very similar
properties. The table is also divided into rows, known as periods.
The periodic table groups the elements based on their properties.
Chemical Reactions
A molecule is any combination of two or more atoms. The oxygen in the air we breathe is two oxygen atoms
connected by a chemical bond to form O2 , or molecular oxygen. A carbon dioxide molecule is a combination of one
carbon atom and two oxygen atoms. Because carbon dioxide includes two different elements it is a compound as
well as a molecule.
A compound is any combination of two or more elements. A compound has different properties from the elements
that it contains. Elements and combinations of elements make up all the many types of matter in the universe. A
chemical reaction is a process that breaks or forms the bonds between atoms.
For example, hydrogen and oxygen bind together to form water. The molecules that come together to start a chemical
reaction are the reactants. So hydrogen and oxygen are reactants. The product is the end result of a reaction. In
this example, water is the product.
Organic Compounds
The chemical components of living organisms are known as organic compounds. Organic compounds are molecules
built around the element carbon (C). Living things are made up of very large molecules. These large molecules are
called macromolecules because “macro” means large. Our body gets the organic molecules we need from the food
we eat (Figure 2.8 ). Which organic molecules do you recognize from the list below?
The four main macromolecules found in living things, shown in Table 2.1 , are:
Nucleic Acids
What are proteins and what do they do? can be seen at
What is DNA? can be viewed at
A healthy diet includes protein fat and
carbohydrate providing us with organic
TABLE 2.1: The Four Main Classes of Organic Molecules
Enzymes, muscle
fibers, antibodies
Glycogen, Cellulose
Amino acids
Often include fatty
oils, waxes, steroids
Often include fatty
Nucleic Acids
|+ The Four Main Classes of Organic Molecules
The Molecules of Cells, an overview of the molecules of the cell, can be viewed at
?v=Q1dRmbCCO4Y#38;feature=fvw (6:09).
Click image to the left for more content.
Carbohydrates are sugars or long chains of sugars. An important role of carbohydrates is to store energy. Glucose
(Figure 2.9 is a simple sugar molecule with the chemical formula C6 H12 O6 .
A molecule of glucose a carbohydrate.
Carbohydrates also include long chains of connected sugar molecules. Plants store sugar in long chains called starch,
whereas animals store sugar in long chains called glycogen. You get the carbohydrates you need for energy from
eating carbohydrate-rich foods, including fruits and vegetables, as well as grains, such as bread, rice, or corn.
Proteins are molecules that have many different functions in living things. All proteins are made of small molecules
called amino acids that connect together like beads on a necklace (Figure 2.10 ) and Figure 2.12 ). There are only
20 common amino acids needed to build proteins. These amino acids form in thousands of different combinations,
making 100,000 or more unique proteins in humans. Proteins can differ in both the number and order of amino
acids. Small proteins have just a few hundred amino acids. The largest proteins have more than 25,000 amino acids.
Many important molecules in your body are proteins. Enzymes are a type of protein that speed up chemical reactions. For example, your stomach would not be able to break down food if it did not have special enzymes to speed
Amino Acids connect together like
beads on a necklace.
up the rate of digestion. Antibodies that protect you against disease are proteins. Muscle fiber is mostly protein
(Figure 2.11 ).
Muscle fibers are made mostly of protein.
General Structure of Amino Acids. This
model shows the general structure of
all amino acids. Only the side chain R
varies from one amino acid to another.
KEY H ¯ hydrogen N ¯ nitrogen C ¯ carbon O ¯ oxygen R ¯ variable side chain.
It’s important for you and other animals to eat food with protein because we cannot make some amino acids ourselves. You can get proteins from plant sources, such as beans, and from animal sources, like milk or meat. When
you eat food with protein, your body breaks the proteins down into individual amino acids and uses them to build
new proteins. You really are what you eat!
Have you ever tried to put oil in water? They don’t mix. Oil is a type of lipid. Lipids are molecules such as fats,
oils, and waxes. The most common lipids in your diet are probably fats and oils. Fats are solid at room temperature,
whereas oils are fluid. Animals use fats for long-term energy storage and to keep warm. Plants use oils for long-term
energy storage. When preparing food, we often use animal fats, such as butter, or plant oils, such as olive oil or
canola oil.
There are many more type of lipids that are important to life. One of the most important are the phospholipids (see
the chapter titled Cell Functions) that make up the protective outer membrane of all cells (Figure 2.13 ).
Phospholipids in a membrane.
Nucleic acids
Nucleic acids are long chains of nucleotides. Nucleotides are made of a sugar, a nitrogen-containing base, and a
phosphate group. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two main nucleic acids. DNA is
the molecule that stores our genetic information (Figure 2.14 ). RNA is involved in making proteins. ATP (adenosine
triphosphate), known as the "energy currency" of the cell, is also a nucleic acid. See the National Institutes of Health
publication The New Genetics ( for further information.
DNA a nucleic acid.
An overview of DNA can be seen at
Click image to the left for more content.
Arsenic in Place of Phosphorus?
In late 2010, scientists proposed that the notion that the elements essential for life - carbon, hydrogen, oxygen,
nitrogen, phosphorus and sulfur - may have additional members. Scientists have trained a bacterium to eat and grow
on a diet of arsenic, in place of phosphorus. Phosphorus chains form the backbone of DNA, and ATP is the principal
molecule in which energy in the cell is stored. Arsenic is directly under phosphorus in the Periodic Table, so the two
elements have similar chemical bonding properties. This finding raises the possibility that organisms could exist on
Earth or elsewhere in the universe using biochemicals not currently known to exist. These results will expand the
notion of what life could be and where it could be. It could be possible that life on other planets may have formed
using biochemicals with other elements.
See;_r=3 for further information.
Lesson Summary
Elements are substances that cannot be broken down into simpler substances with different properties.
Elements have been organized by their properties to form the periodic table.
Two or more atoms can combine to form a molecule.
Molecules consisting of more than one element are called compounds.
Reactants can combine through chemical reactions to form products.
Enzymes can speed up a chemical reaction.
Living things are made of just four classes of macromolecules: proteins, carbohydrates, lipids, and nucleic
Review Questions
1. What are the 4 main classes of organic compounds?
2. Sugar is what kind of organic compound?
3. What is an atom?
4. Name a few examples of proteins.
5. Name a few examples of lipids in organisms.
6. What are two nucleic acids?
Apply Concepts
7. Would water, with the symbol H2 O, be considered an element or a compound?
8. How many types of atoms make up gold?
Think Critically
9. Why do you think you need fats in your diet?
Points to Consider
• Do you expect the genetic information in the DNA of a cow to be the same or different from that in a crow?
• If we are all composed of the same chemicals, how do all organisms look so different?
• What characteristics would you use to distinguish and classify living things?
Classification of Living Things
Lesson Objectives
Explain what makes up a scientific name.
Explain what defines a species.
List the information scientists use to classify organisms.
List the three domains of life and the chief characteristics of each.
Check Your Understanding
• What are the basic characteristics of life?
• What are the four main classes of organic molecules that are building blocks of life?
Archaea Microscopic one-celled organisms with no nucleus that tend to live in extreme environments.
bacteria Microscopic one-celled prokaryotic organisms (without a nucleus).
binomial nomenclature The system for naming species in which the first word is the genus and the second word
is the species.
classify To organize into groups or categories; scientists classify organisms by their physical features and how
closely related they are.
domain The least specific category of classification.
Eukarya Domain in which cells have a nucleus; includes plants, animals, fungi, and protists.
genus The first word in the two word name given to every organism.
species A group of individuals that are genetically related and can breed to produce fertile young; the second word
in the two word name given to every organism is the species name.
taxonomy The science of naming and classifying organisms.
Classifying Organisms
When you see an organism that you have never seen before, you probably put it into a group without even thinking.
If it is green and leafy, you probably call it a plant. If it is long and slithers, you probably call it as a snake. How do
you make these decisions? You look at the physical features of the organism and think about what it has in common
with other organisms.
Scientists do the same thing when they classify, or put in categories, living things. Scientists classify organisms not
only by their physical features, but also by how closely related they are. Lions and tigers look like each other more
than they look like bears. It turns out that the two cats are actually more closely related to each other than to bears.
How an organism looks and how it is related to other organisms determines how it is classified.
Linnaean system of classification
People have been concerned with classifying organisms for thousands of years. Over 2,000 years ago, the Greek
philosopher Aristotle developed a classification system that divided living things into several groups that we still use
today, including mammals, insects, and reptiles.
Carl Linnaeus (1707-1778) (Figure ?? ) built on Aristotle’s work to create his own classification system. He invented
the way we name organisms today. Linnaeus is considered the inventor of modern taxonomy, the science of naming
and grouping organisms. See for additional information.
Linnaeus developed binomial nomenclature, a way to give a scientific name to every organism. Each species
receives a two-part name in which the first word is the genus (a group of species) and the second word refers to one
species in that genus. For example, a coyote’s species name is Canis latrans. Latrans is the species and canis is the
genus, a larger group that includes dogs, wolves, and other dog-like animals.
Here is another example: the red maple, Acer rubra, and the sugar maple, Acer saccharum, are both in the same
genus and they look similar (Figure 2.15 , Figure 2.16 , and Figure 2.17 ). Notice that the genus is capitalized and
the species is not, and that the whole scientific name is in italics. The names may seem strange, but they are written
in a language called Latin.
Modern Classification
Modern taxonomists have reordered many groups of organisms since Linnaeus. The main categories that biologists
use are listed here from the most specific to the least specific category (Figure 2.18 ). See
nova/orchid/classifying.html for further information.
• Least Specific
These leaves are from one of two different species of trees in the Acer or maple
These leaves are from one of two different species of trees in the Acer or maple
One of the characteristics of the maple
genus is winged seeds.
• Most Specific
This diagram illustrates the classification categories for organisms with the
broadest category Kingdom at the bottom and the most specific category
Species at the top.
The Classification Rap can be heard at (3:18).
Click image to the left for more content.
Difficulty Naming Species
Even though naming species is straightforward, deciding if two organisms are the same species can sometimes be
difficult. Linnaeus defined each species by the distinctive physical characteristics shared by these organisms. But
two members of the same species may look quite different. For example, people from different parts of the world
sometimes look very different, but we are all the same species (Figure 2.19 ).
So how is a species defined? A species is group of individuals that can interbreed with one another and produce
fertile offspring; a species does not interbreed with other groups. By this definition, two species of animals or
plants that do not interbreed are not the same species. See Biological Classification of Organisms for additional
These children are all members of the
same species Homo sapiens.
Domains of Life
Let’s explore the least specific category of classification, called a domain.
All of life can be divided into 3 domains, which tell you the type of cell inside of an organism:
a. Bacteria: Single-celled organisms that do not contain a nucleus
b. Archaea: Single-celled organisms that do not contain a nucleus; have a different cell wall from bacteria
c. Eukarya: Organisms with cells that contain a nucleus.
Archaea and Bacteria
Archaea and Bacteria (Figure 2.20 and Figure 2.21 ) seem very similar, but they also have significant differences.
Do not have a nucleus
Small cells
Can reproduce without sex by dividing in two
• Cell walls made of different material
• Archaea often live in extreme environments like hot springs, geysers, and salt flats while bacteria can live
almost everywhere.
The Group D Streptococcus organism is
in the domain Bacteria one of the three
domains of life.
All of the cells in the domain Eukarya keep their genetic material, or DNA, inside the nucleus. The domain Eukarya
is made up of four kingdoms:
a. Plantae: Plants, such as trees and grasses, survive by capturing energy from the sun, a process called photosynthesis.
b. Fungi: Fungi, such as mushrooms and molds, survive by "eating" other organisms or the remains of other
c. Animalia: Animals survive by eating other organisms or the remains of other organisms. Animals range from
tiny worms to insects, dogs, and the largest dinosaurs and whales (Figure 2.22 ).
d. Protista: Protists are not all descended from a single common ancestor in the way that plants, animals, and
fungi are. Protists are all the eukaryotic organisms that do not fit into one of the other three kingdoms. They
The Halobacterium is in the domain Archaea one of the three domains of life.
include many kinds of microscopic one-celled organisms, such as algae and plankton, but also giant seaweeds
that can grow to be 200 feet long (an alga protist is shown in Figure 2.23 ).
Plants, animals, fungi, and protists might seem very different, but remember that if you look through a microscope,
you will find similar cells with a membrane-bound nucleus in all of them. The main characteristics of the three
domains of life are summarized in Table 2.2 .
The Western Gray Squirrel is in the domain Eukarya one of the three domains
of life.
TABLE 2.2: Three domains of life: Bacteria, Archaea, and Eukarya
Cell Wall
Nucleus (DNA inside a
Yes, without peptidoglycan
Yes, with peptidoglycan
Varies. Plants and fungi
have a cell wall; animals
do not.
TABLE 2.2: (continued)
Organelles inside a membrane
|+Three domains of life: Bacteria, Archaea, and Eukarya
We have all heard of viruses. The flu and many other diseases are caused by viruses. But what is a virus? Based on
the material presented in this chapter, do you think viruses are living?
The answer is actually “no.” A virus is essentially DNA or RNA surrounded by a coat of protein (Figure ?? ). It
is not a cell and does not maintain homeostasis. Viruses also cannot reproduce on their own – they need to infect a
host cell to reproduce. Viruses do, however, change over time, or evolve. So a virus is very different from any of the
organisms that fall into the three domains of life.
Lesson Summary
• Scientists have defined several major categories for classifying organisms: domain, kingdom, phylum, class,
order, family, genus, and species.
• The scientific name of an organism consists of its genus and species.
• Scientists classify organisms according to their evolutionary histories and how related they are to one another
- by looking at their physical features, the fossil record, and DNA sequences.
• All life can be classified into three domains: Bacteria, Archaea, and Eukarya.
Review Questions
1. Who designed modern classification and invented the two-part species name?
2. Define a species.
3. What kingdoms make up the domain Eukarya?
4. What is the name for the scientific study of naming and classifying organisms?
5. How are organisms given a scientific name?
Apply Concepts
6. In what domain are humans?
7. Quercus rubra is the scientific name for the red oak tree. What is the red oak’s genus?
8. In what domain are mushrooms?
This microscopic alga is a protist in the
domain Eukarya.
9. What information do scientists use to classify organisms?
Think Critically
10. Is it possible for organisms in two different classes to be in the same genus?
11. If molecular data suggests that two organisms have very similar DNA, what does that say about their evolutionary
12. Can two different species ever share the same scientific name?
13. If two organisms are in the same genus, would you expect them to look much alike?
Points to Consider
• This Section introduced the diversity of life on Earth. Do you think it is possible for cells from different
organisms to be similar even though the organisms look different?
• Do you think human cells are different from bacterial cells?
• Do you think it is possible for a single cell to be a living organism?
MS Cells and Their Structures
Look carefully at the above image. What do you see? What colors? What shapes? Can you guess what it is?
These are actually cells from a dog’s kidney. Cells are the smallest units of living things. You are made of cells.
Plants are made of cells. So are dogs, chickens, bees and mushrooms. Our cells even look very similar to the above
dog cells.
But the cells of a kidney are not actually bright green and red. Scientists remove cells from organisms, dye them
with different colors, and look at them under strong microscopes. The above image is an example of what scientists
see under these microscopes. Now, we will explore what different types of cells look like and what they do.
Introduction to Cells
Lesson Objectives
• Explain how cells are observed.
• Define the three main parts of the cell theory.
• Explain the levels of organization in an organism.
Check Your Understanding
• What are the five main characteristics of living things?
• Name the four main classes of organic molecules that are building blocks of life.
organ A group of tissues that work together to perform a common function.
organ system A group of organs that work together to perform a common function.
tissue A group of specialized cells that function together.
What are cells?
In the chapter What is a Living Organism?, you learned that living things are made of big molecules called proteins,
lipids, carbohydrates, and nucleic acids. When these big molecules come together, they form a cell. A cell is the
smallest unit of an organism that is still considered living (see the onion cells in Figure 3.1 ). Some organisms, like
bacteria, consist of only one cell. Big organisms, like humans, consist of trillions of cells. Compare a human to a
banana. On the outside, they look very different, but if you look close enough you’ll see that their cells are actually
very similar.
The Inner Life of the Cell can be viewed at (5:28).
Click image to the left for more content.
The outline of onion cells are visible under a light microscope.
Observing Cells
Most cells are so tiny that you cannot see them without the help of a microscope. It was not until 1665 that English
scientist Robert Hooke invented a basic light microscope and observed cells for the first time. You may use light
microscopes in the classroom. You can use a light microscope to see cells. But many structures in the cell are too
small to see with a light microscope. So, what do you do if you want to see the tiny structures inside of cells?
In the 1950s, scientists developed more powerful microscopes. A light microscope sends a beam of light through a
specimen, or the object you are studying. A more powerful microscope, called an electron microscope, passes a
beam of electrons through the specimen. Sending electrons through a cell allows us to see its tiniest parts (Figure
3.2 ).
Without electron microscopes, we would not know what the inside of a cell looked like. The only problem with
using an electron microscope is that it only works with dead cells. Scientists and students still use light microscopes
to study living cells.
An electron microscope allows scientists to see much more detail than a
light microscope as with this sample of
pollen. But a light microscope allows scientists to study living cells.
How to Correctly Use a Microscope can be viewed at;feature=
related (1:43).
Click image to the left for more content.
Cell Theory
In 1858, after using microscopes much better than Hooke’s first microscope, Rudolf Virchow developed the hypothesis that cells only come from other cells. For example, bacteria are composed of only one cell (Figure 3.3 ) and
divide in half to make new bacteria. In the same way, your body makes new cells by dividing the cells you already
have. In all cases, cells only come from cells that have existed before. This idea led to the development of one of
the most important theories in biology, cell theory.
Cell theory states that:
a. All organisms are composed of cells.
b. Cells are alive and the basic living units of organization in all organisms.
c. All cells come from other cells.
As with other scientific theories, many hundreds, if not thousands, of experiments support the cell theory. Since
Virchow created the theory, no evidence has ever contradicted it.
Bacteria pink are an example of an organism consisting of only one cell.
Levels of Organization
Although cells share many of the same features and structures, they also can be very different. Each cell in your
body is designed for a specific task.
For example:
• Red blood cells (Figure 3.4 ) are shaped with a pocket that traps oxygen and brings it to other body cells.
• Nerve cells, which can quickly send the feeling of touching a hot stove to your brain, are long and stringy in
order to form a line of communication with other nerve cells, like a wire (Figure 3.5 ).
• Skin cells (Figure 3.6 ) are flat and fit tightly together to protect your body.
An animation comparing the size of red blood cells and skin cells to other structures can be found at http://learn.gen
As you can see, cells are shaped in ways that help them do their jobs. Multicellular (many-celled) organisms have
many types of specialized cells in their bodies.
Red blood cells are specialized to carry
oxygen in the blood.
While cells are the basic units of an organism, groups of cells can be specialized, or perform a specific job. Specialized cells can be organized into tissues. For example, your liver cells are organized into liver tissue, which is
organized into an organ, your liver. Organs are formed from two or more specialized tissues working together to
perform a job that helps your body work. All organs, from your heart to your liver, are made up of an organized
group of tissues.
These organs are part of a larger system, the organ systems. For example, your brain works together with your spinal
cord and other nerves to form the nervous system. This organ system must be organized with other organ systems,
such as the circulatory system and the digestive system, for your body to work. Organ systems work together to
form the entire organism. As you can see (Figure 3.7 ), there are many levels of organization in living things. See for additional information.
Lesson Summary
• Cells were first observed under a light microscope, but today’s electron microscopes allow scientists to take a
closer look at the inside of cells.
Neurons are shaped to conduct electrical impulses to many other nerve cells.
These epidermal cells make up the
&#8220 skin&#8221 of plants.
how the cells fit tightly together.
Levels of Organization from the atom to
the organism.
• Cell theory says that:
– All organisms are composed of cells;
– Cells are alive and the basic living units of organization in all organisms; and
– All cells come from other cells.
• Cells are organized into tissues, which are organized into organs, which are organized into organ systems,
which are organized to create the whole organism.
Review Questions
1. What scientific tool was used to first observe cells?
2. What are the three main parts of the cell theory?
Apply Concepts
3. Put the following in the correct order from simplest to most complex: organ, cell, tissue, organ system.
4. What type of microscope would be best for studying the structures found inside of cells?
Think Critically
5. According to the cell theory, can we create a new cell in laboratory by putting different molecules together? Why
or why not?
Further Reading / Supplemental Links
• Baeuerle, Patrick A. and Landa, Norbert. The Cell Works: Microexplorers. Barron’s; 1997, Hauppauge, New
• Sneddon, Robert. The World of the Cell: Life on a Small Scale. Heinemann Library; 2003, Chicago.
• Wallace, Holly. Cells and Systems. Heinemann Library; 2001, Chicago.
Points to Consider
• Do you think there would be a significant difference between bacterial cells and your brain cells? What might
they be?
• Do you think a bacterial cell and a brain cell have some things in common? What might they be?
• Do you think cells have organs like we do? How would that benefit cells?
Cell Structures
Lesson Objectives
Compare prokaryotic and eukaryotic cells.
List the organelles of the cell and their functions.
Discuss the structure and function of the cell membrane and cytosol.
Describe the structure and function of the nucleus.
Distinguish between plant and animal cells.
Check Your Understanding
• What is a cell?
• How do we visualize cells?
cell wall Provides strength and protection for the cell; found around plant, fungal, and bacterial cells.
central vacuole Large organelle containing water, nutrients, and wastes; can take up to 90% of a plant cell’s
chloroplast The organelle in which photosynthesis takes place.
chromosome DNA wound around proteins; forms during prophase of mitosis and meiosis.
cytoplasm All the contents of the cell besides the nucleus, including the cytosol and the organelles.
cytoskeleton The internal scaffolding of the cell; maintains the cell shape and aids in moving the parts of the cell.
cytosol A fluid-like substance inside the cell; organelles are embedded in the cytosol.
endoplasmic reticulum (ER) A folded membrane organelle; rough ER modifies proteins and smooth ER makes
eukaryote Cell belonging to the domain Eukarya (fungi, animals, protists, and plants); has membrane-enclosed
nucleus and organelles.
Golgi apparatus The organelle where proteins are modified, labeled, packaged into vesicles, and shipped.
lysosome Organelle which contains degradative enzymes; breaks down unneeded materials.
mitochondria Organelle where cellular respiration occurs; known as the "powerhouse" of the cell because this is
the organelle where the ATP that powers the cell is produced.
nuclear envelope A double membrane that surrounds the nucleus; helps regulate the passage of molecules in and
out of the nucleus.
nucleus Membrane enclosed organelle in eukaryotic cells that contains the DNA; primary distinguishing feature
between a eukaryotic and prokaryotic cell; the information center, containing instructions for making all the
proteins in a cell, as well as how much of each one.
organelle Small structure found in cells; has specialized functions; many are membrane-bound, such as mitochondria, plastids, and vacuoles. Membrane-bound organelles are found only in eukaryotic cells.
plasma membrane Surrounds the cell; made of a double layer of specialized lipids, known as phospholipids,
with embedded proteins; regulates the movement of substances into and out of the cell; also called the cell
prokaryote A microscopic single-celled organism, including bacteria and cyanobacteria; does not have a nucleus
with a membrane or other specialized organelles.
ribosome The cell structure on which proteins are made; not surrounded by a membrane; found in both prokaryotic
and eukaryotic cells.
rough endoplasmic reticulum The part of the ER with ribosomes attached; proteins can be modified in the rough
ER before they are packed into vesicles for transport to the golgi apparatus.
semipermeable Allowing only certain materials to pass through; characteristic of the cell membrane.
smooth endoplasmic reticulum Part of the ER that does not have ribosomes attached; where lipids are synthesized.
vesicle Small membrane-enclosed sac; transports proteins around a cell or out of a cell.
Prokaryotic and Eukaryotic Cells
There are two basic types of cells, prokaryotic cells (Figure 3.8 ), found in organisms called prokaryotes, and
eukaryotic cells (Figure 3.9 ), found in organisms called eukaryotes.
The main difference between eukaryotic and prokaryotic cells is that eukaryotic cells have a nucleus, where they
store their DNA, or genetic material. The nucleus is membrane-bound, which means it is surrounded by a phospholipid membrane. Prokaryotic cells do not have a "membrane-bound" nucleus. Instead, their DNA floats around
inside the cell.
Here are some other key features of eukaryotic cells:
a. They have membrane-bound structures called organelles. A list of the main eukaryotic organelles is located
in Table 3.2 .
b. Eukaryotic cells include the cells of fungi, animals, protists, and plants.
c. These cells are more specialized than prokaryotic cells.
Key features of prokaryotic cells include:
The cells are usually smaller and simpler than eukaryotic cells.
Prokaryotic cells do not have membrane-bound structures.
The DNA, or genetic material, forms a single large circle that coils up on itself.
Prokaryotic cells belong to the domains Bacteria or Archaea. These two domains were discussed in the What
is a Living Organism? chapter.
From the above information, are the cells found in your body prokaryotic cells or eukaryotic cells? Table 3.1
compares prokaryotic and eukaryotic cells.
Prokaryotes do not have a nucleus. Instead their genetic material is a simple
loop of DNA.
TABLE 3.1: Comparison of Prokaryotic and Eukaryotic Cells
Membrane-enclosed organelles
Prokaryotic cells
Single “naked” circle; plasmids
Eukaryotic cells
In membrane-enclosed nucleus
Plants, animals, fungi
An Introduction to Prokaryotic and Eukaryotic Cells can be viewed at
G94DO0 (2:13)
Eukaryotic cells contain a nucleus
where the DNA lives and surrounded by a membrane
and various other special compartments
surrounded by membranes called organelles. For example notice in this
image the mitochondria
and peroxisomes.
Click image to the left for more content.
The Plasma Membrane and Cytosol
Both eukaryotic and prokaryotic cells have walls around them that separate them from other cells and make sure the
parts of the cell do not just float away. This wall is called a plasma membrane. The plasma membrane is made
of a double layer of lipids, known as phospholipids. The function of the plasma membrane, also known as the cell
membrane, is to control what goes in and out of the cell.
Some molecules can go through the cell membrane and enter and leave the cell, but some cannot. "Permeable"
means that anything can cross a barrier. An open door is completely permeable to anything that wants to enter or
exit through the door. The plasma membrane is semipermeable, meaning that some things can enter the cell and
some things cannot.
The inside of eukaryotic and prokaryotic cells also both contain a jelly-like substance called cytosol. Cytosol is
composed of water and other molecules, including enzymes that speed up the cell’s chemical reactions. Everything
in the cell - the nucleus and the organelles - sit in the cytosol, like fruit in a Jell-o mold. The term cytoplasm refers
to the cytosol and all of the organelles, but not the nucleus.
TABLE 3.2: Some Eukaryotic Organelles
Golgi apparatus
Smooth Endoplasmic Reticulum
Involved in making proteins
Packages proteins and some polysaccharides
Where ATP is made
Makes lipids
TABLE 3.2: (continued)
Makes sugar (photosynthesis)
Digests macromolecules
The Nucleus
The nucleus is only found in eukaryotic cells. It is a membrane-bound structure that contains most of the genetic
material of the cell (Figure ?? ). The nucleus contains important information that helps the cell create important
molecules for life. The nuclear envelope, a double membrane that surrounds the nucleus, controls which molecules
go in and out of the nucleus.
Inside of the nucleus, you will find the chromosomes. Chromosomes are strands of DNA wrapped around proteins.
They contain genes, or small units of genetic material that create proteins.
Organelles in the Cytoplasm: The Cell Factory
A cell is like a factory. Just as a factory is made up of many people and machines, a cell has many different parts,
each with a special role. The different parts of the cell are called organelles, which means "small organs." All
organelles are found in eukaryotic cells, but most are NOT found in prokaryotic cells. Pay attention to which ones
are included in prokaryotic cells.
Below are the main organelles found in cells:
a. The nucleus of a cell is like a safe containing the factory’s trade secrets, including information about how to
build thousands of proteins.
b. The mitochondria are powerhouses that create ATP (adenosine triphosphate), which provides the energy
needed to power chemical reactions. Plant cells have special organelles called chloroplasts that capture energy
from the sun and store it in the bonds of sugar molecules, using a process called photosynthesis (Figure 3.10
). (The cells of animals and fungi do not photosynthesize and do not have chloroplasts.)
c. The vacuoles are like storage centers. Plant cells have larger ones than animal cells because they need to store
water and other nutrients.
d. The lysosomes are like the recycling trucks that carry waste away from the factory. Inside lysosomes are
enzymes that break down old molecules into parts that can be recycled into new ones.
e. Eukaryotic cells also contain a skeleton-like structure called the cytoskeleton. Like our bony skeleton, a
cell’s cytoskeleton gives the cell its shape and helps the cell to move. What part of a factory would act like a
f. In both eukaryotes and prokaryotes, ribosomes are where proteins are made. Ribosomes are like the machines
in the factory that produce the factory’s main product. Proteins are the main product of the cell.
g. Some ribosomes can be found on folded membranes called the endoplasmic reticulum (ER). If the ER is
covered with ribosomes, it looks bumpy and is called rough endoplasmic reticulum. If the ER does not
contain ribosomes, it is smooth and called the smooth endoplasmic reticulum. Proteins are made on the
rough ER. Lipids are made on the smooth ER.
h. The Golgi apparatus, works like a mail room. The Golgi apparatus receives the proteins from the rough ER,
puts "shipping addresses" on the proteins, packages them up in vesicles, and then sends them to the right place
in the cell.
You can watch an interactive animation of plant and animal cells at the following link:
Diagram of chloroplast a and electron
microscope image of two mitochondria
b. Chloroplasts and mitochondria provide energy to cells. If the bar at the bottom of the electron micrograph image is
200 nanometers what is the diameter of
one of the mitochondria
Differences between Plant and Animal Cells
Even though plants and animals are both eukaryotes, plant cells differ in some ways from animal cells. First, plant
cells have a large central vacuole that holds a mixture of water, nutrients, and wastes. A plant cell’s vacuole can
make up 90% of the cell’s volume. In animal cells, vacuoles are much smaller.
Second, plant cells have a cell wall, while animal cells do not. A cell wall gives the plant cell strength and protection.
A third difference between plant and animal cells is that plants have several kinds of organelles called plastids. There
are several kinds of plastids, including chloroplasts, needed for photosynthesis; leucoplasts, which store starch and
oil; and brightly colored chromoplasts, which give some flowers and fruits their yellow, orange, or red color. You
will learn more about chloroplasts and photosynthesis in the chapter titled Cell Functions. Under a microscope one
can see plant cells more clearly (Figure 3.11 and Figure 3.12 ).
A plant cell has several features that
make it different from an animal cell
including a cell wall huge vacuoles
and several kinds of plastids including
chloroplasts which photosynthesize.
In this photo of plant cells taken with
a light microscope you can see a cell
wall purple around each cell and green
Lesson Summary
• Prokaryotic cells lack a nucleus; eukaryotic cells have a nucleus.
• Each component of a cell has a specific function.
• Plant cells are different from animal cells. For example, plant cells contain plastids, cell walls, and large
Review Questions
1. What are the two basic types of cells?
2. What are organelles?
3. Discuss the main differences between prokaryotic cells and eukaryotic cells.
Apply Concepts
4. What is the plasma membrane and what is its role?
5. Why is the mitochondria known as the powerhouse of the cell?
Think Critically
6. Why does photosynthesis not occur in animal cells?
Further Reading / Supplemental Links
• Baeuerle, Patrick A. and Landa, Norbert. The Cell Works: Microexplorers. Barron’s; 1997, Hauppauge, New
• Sneddon, Robert. The World of the Cell: Life on a Small Scale. Heinemann Library; 2003, Chicago.
• Wallace, Holly. Cells and Systems. Heinemann Library; 2001, Chicago.
Points to Consider
• Think about what molecules would need to be transported into cells.
• Discuss why you think it would be important for some molecules to be kept out of a cell.
MS Cell Functions
Multi-celled organisms, like dolphins, are made up of trillions of cells. How do you think they work together to
move an organism? How do the cells of a tree allow it to absorb water and produce leaves? How are the cells
interacting with the world inside of the body and outside of the body? Do small one-celled organisms function the
same way as the cells in big organisms like dolphins? We need to know how cells function, so we can understand
how entire organisms, both large and small, function.
Lesson Objectives
• Describe methods of transporting molecules into and out of the cell.
• Distinguish between active and passive transport.
• Explain how diffusion and osmosis work.
Check Your Understanding
• What structure surrounds the cell?
• What is the primary part of the cell membrane?
• What does homeostasis mean?
active transport The movement of a molecule from an area of lower concentration to an area of higher concentration; requires a carrier protein and energy.
concentration The amount of a substance in relation to the volume.
diffusion Movement of molecules from an area of high concentration to an area of low concentration; does not
require energy.
hypertonic solution Having a higher solute concentration than the cell; cell will lose water by osmosis.
hypotonic solution Having a lower solute concentration than the cell; cell will gain water by osmosis.
isotonic solution A solution in which the amount of dissolved material is equal both inside and outside the cell;
no net gain or loss of water.
osmosis Diffusion of water across a membrane.
passive transport Movement of molecules from an area of higher concentration to an area of lower concentration;
does not require energy.
phospholipid A lipid molecule with a hydrophilic head and two hydrophobic tails; makes up the cell membrane.
selectively permeable Semipermeable; property of allowing only certain molecules to pass through the cell membrane.
Cells are found in all different types of environments, and these environments are constantly changing. One-celled
organisms, like bacteria, can be found on your skin, or in the ground, or in all different types of water. The cells
of your body interact with the food you eat, and also with other cells in your body. All cells need a way to protect
themselves. This job is done by the cell membrane.
The cell membrane is semipermeable, or selectively permeable, which means that only some molecules can get
through the membrane. If the cell membrane were completely permeable, the inside of the cell would be the same as
the outside of the cell. It would be impossible for the cell to maintain homeostasis. Homeostasis means maintaining
a stable internal environment. For example, if your body cells have a temperature of 98.6 degrees F, and it is freezing
outside, your cells will maintain homeostasis if the temperature of the cells stays the same and does not drop.
How does the cell ensure it is semipermeable? How does the cell control what molecules enter and leave the cell?
The ways that cells control what passes through the cell membrane will be the focus of this lesson.
What is Transport?
Molecules in the cell membrane allow it to be semipermeable. The membrane is made of a double layer of phospholipids (a "bilayer") and proteins (Figure 4.1 ). A single phospholipid molecule has two parts:
a. A head that is hydrophilic, or water-loving.
b. A tail that is hydrophobic, or water-fearing.
There is water found on both the inside and the outside of cells. Since hydrophilic means water-loving and they
want to be near water, the heads face the inside and outside of the cell where water is found. The water-fearing,
hydrophobic tails face each other in the middle of the cell membrane because water is not found in this space. An
interesting quality of the plasma membrane is that it is constantly moving, like a soap bubble. Water and small
molecules such as oxygen and carbon dioxide can pass freely through the membrane, but larger molecules cannot
easily pass through the plasma membrane. Some molecules need a special way to get across the membrane.
The cell membrane is made up of a
phospholipid bilayer two layers of phospholipid molecules.
Small molecules can pass through the plasma membrane through a process called diffusion. Diffusion is the movement of molecules from an area where there is a higher concentration (larger amount) of the substance to an area
where there is a lower concentration (lower amount) of the substance (Figure 4.2 ). The amount of a substance in
relation to the total volume is the concentration.
Diffusion is the movement of a substance from an area of a higher amount
towards an area of lower amount. Equilibrium is reached when there is an
equal amount on both sides of the membrane.
The diffusion of water across a membrane because of a difference in concentration is called osmosis. Let’s explore
three different situations and analyze the flow of water.
a. A hypotonic solution means the environment outside of the cell has a lower concentration of dissolved material than the inside of the cell. If a cell is placed in a hypotonic solution, water will move into the cell. This
causes the cell to swell, and it may even burst.
b. A hypertonic solution means the environment outside of the cell has more dissolved material than inside of
the cell. If a cell is placed in a hypertonic solution, water will leave the cell. This can cause a cell to shrink
and shrivel.
c. An isotonic solution is a solution in which the amount of dissolved material is equal both inside and outside
of the cell. Water still flows in both directions, but an equal amount enters and leaves the cell.
How do marine animals keep their cells from shrinking? How do blood cells keep from bursting? Both have to
do with the cell membrane and transport of materials. Marine animals live in salt water, which is a hypertonic
environment; there is more salt in the water than in their cells. To prevent losing too much water from their bodies,
these animals intake large quantities of salt water and secrete salt by active transport, which will be discussed later
in this lesson. Red blood cells can be kept from bursting or shriveling if put in a solution that is isotonic to the blood
cells. If the blood cells were put in pure water, the solution would be hypotonic to the blood cells, so water would
enter the blood cells and they would swell and burst. This is represented in Figure 4.3 .
Passive Transport
Diffusion is called passive transport. This means it does not require energy to move molecules. For example,
oxygen diffuses out of the air sacs in your lungs into your bloodstream because oxygen is more concentrated in
your lungs than in your blood. Oxygen moves from the high concentration of oxygen in your lungs to the low
concentration of oxygen in your bloodstream. Sometimes, special proteins are needed to help molecules move
across the membrane. These are called channel proteins or carrier proteins (Figure 4.4 ).
Osmosis causes these red blood cells to
change shape by losing or gaining water.
Protein channels and carrier proteins
are involved in passive transport.
Diffusion Across Cell Membranes: Passive Transport can be viewed at
BWGMyY (4:41).
Click image to the left for more content.
Active Transport
During active transport, molecules move from an area of low concentration to high concentration. This is the
opposite of diffusion. Active transport is called "active" because this type of transport requires energy to move
molecules. A protein in the membrane carries the molecules across the membrane. These proteins are often called
"pumps", because like other pumps they use energy to move molecules. There are many cells in your body that use
pumps to move molecules. For example, your nerve cells would not send messages to your brain unless you had
protein pumps moving molecules by active transport. The sodium-potassium pump (Figure 4.5 ) is an example of
an active transport pump.
An overviewof active transport can be viewed at;feature=rela
ted (1:26).
Click image to the left for more content.
The sodium-potassium pump moves
sodium ions to the outside of the cell
and potassium ions to the inside of the
ATP is required for the protein to
change shape. As ATP adds a phosphate group to the protein it leaves behind adenosine diphosphate ADP.
Lesson Summary
• The plasma membrane is semipermeable, meaning that some molecules can move through the membrane
easily, while others cannot.
• Passive transport, such as diffusion and osmosis, does not require energy.
• Active transport moves molecules in the direction of the higher concentration and requires energy and a carrier
Review Questions
1. What’s the main difference between active and passive transport?
2. List the two types of passive transport.
3. Why is the plasma membrane considered semipermeable?
4. What is diffusion?
Apply Concepts
5. What happens when a cell is placed in a hypotonic solution?
6. What happens when a cell is placed in a hypertonic solution?
Critical Thinking
7. If a plant cell is placed in a solution and the cell shrivels up, what type of solution was it placed in? How do you
8. If a there are 100 X molecules on the outside of a cell and 10 X molecules inside of the cell, will X molecules flow
into or out of the cell? Explain why.
Points to Consider
The next lesson discusses photosynthesis.
• It is often said that plants make their own food. What do you think this means?
• What substances do you think would need to move into a leaf cell for the cell to make its own food?
• What substances would need to move out of a leaf cell?
Lesson Objectives
• Explain the importance of photosynthesis.
• Write and interpret the chemical equation for photosynthesis.
• Describe what happens during the light reactions and the Calvin Cycle.
Check Your Understanding
• How are plant cells different from animal cells?
• In what organelle does photosynthesis take place?
chlorophyll Green pigment in leaves; helps to capture solar energy.
photosynthesis The process by which specific organisms (including all plants) use the sun’s energy to make their
own food from carbon dioxide and water; process that converts the energy of the sun, or solar energy, into
carbohydrates, a type of chemical energy.
stomata Special pores in leaves; carbon dioxide enters the leaf and oxygen exits the leaf through these pores.
stroma Fluid in the chloroplast interior space; surrounds the thylakoids.
thylakoid Flattened sacs within the chloroplast; formed by the inner membranes.
What is Photosynthesis?
If a plant gets hungry, it cannot walk to a local restaurant and buy a slice of pizza. So how does a plant get the food it
needs to survive? Photosynthesis is the process plants use to make their own “food” from the sun’s energy, carbon
dioxide and water.
Actually, almost all organisms obtain their energy from photosynthetic organisms. For example, if a bird eats a
caterpillar, then the bird gets the energy that the caterpillar gets from the plants it eats. So the bird is indirectly
getting energy that began with the “food” formed through photosynthesis. Therefore, the process of photosynthesis
is central to sustaining life on Earth.
During photosynthesis, carbon dioxide and water combine with solar energy to create glucose and oxygen. Glucose
is a sugar that acts as the "food" source for plants. Oxygen, which is necessary for animal life, is the waste of
The Photosynthesis Song can be heard at (1:52).
Click image to the left for more content.
The Process of Photosynthesis
Photosynthesis takes place in chloroplasts. Chloroplasts are one of the main differences between plant and animal
cells. There are two separate parts of a chloroplast (Figure 4.6 ).
• The inner compartments formed by the flattened sacs, or thylakoids, are called the thylakoid space. Energy
from sunlight is absorbed by the pigment chlorophyll in the thylakoid membrane.
• The interior space that surrounds the thylakoids is filled with a fluid called stroma. This is where carbon
dioxide is used to produce glucose.
The chloroplast is the photosynthesis
factory of the plant.
The Reactants
What goes into the plant cell? The reactants of photosynthesis are carbon dioxide and water, and the energy from
sunlight. This means that carbon dioxide, water, and the sun’s energy are necessary for the chemical reactions of
• Chlorophyll is the green pigment in leaves that captures energy from the sun.
• The veins in a plant carry water from the roots to the leaves.
• Carbon dioxide enters the leaf from the air through special openings called stomata (Figure 4.7 ).
Stomata are special pores that allow
gasses to enter and exit the leaf.
The Products
What is produced by the plant cell? The products of photosynthesis are glucose and oxygen. This means they are
produced at the end of photosynthesis.
• Glucose, the food of plants, can be used to store energy for later in the form of carbohydrate molecules.
• Oxygen is a plant waste product. It is released into the atmosphere through the stomata. As you know, animals
need oxygen to live. Without photosynthetic organisms like plants, there would not be enough oxygen in the
atmosphere for animals to survive.
The Chemical Reaction
The overall chemical reaction for photosynthesis is 6 molecules of carbon dioxide (CO2 ) and 6 molecules of water
(H2 0), with the addition of solar energy. This produces 1 molecule of glucose (C6 H12 O6 ) and 6 molecules of oxygen
(O2 ) (Figure 4.8 ). Using chemical symbols the equation is represented as follows:
6CO2 + 6H2 O ! C6 H12 O6 + 6O2 .
Lesson Summary
• The net reaction for photosynthesis is that carbon dioxide and water, together with energy from the sun,
produce glucose and oxygen.
As is depicted here the energy from
sunlight is needed to start photosynthesis. The initial steps are called the light
reactions as they occur only in the presence of light. During these initial reactions water is used and oxygen is released. The energy from sunlight is converted into a small amount of ATP and
an energy carrier called NADPH. Together with carbon dioxide these are
used to make glucose sugar through a
process called the Calvin Cycle. NADP
and ADP and Pi inorganic phosphate
are regenerated to complete the process.
A review of photosynthesis can be viewed at (2:41).
Click image to the left for more content.
Review Questions
1. What are the reactants required for photosynthesis?
2. What are the products of photosynthesis?
Apply Concepts
3. What happens to the glucose produced from photosynthesis?
4. Why is it important to animals that oxygen is released during photosynthesis?
5. Describe the structures of the chloroplast where photosynthesis takes place.
Critical Thinking
6. What would happen if the stomata of a plant leaf were glued shut? Would that plant be able to perform photosynthesis? Why or why not?
Supplemental Links
• Bill Nye the Science Guy discusses plants:
Points to Consider
The next lesson is about Cellular Respiration.
• How do you gain energy from the food you eat?
• Which do you think provides more energy- a bowl of pasta or a small piece of candy?
• What “waste” gas do you exhale?
Cellular Respiration
Lesson Objectives
• Write and explain the chemical formula for cellular respiration.
• Explain the two states of cellular respiration.
• Compare photosynthesis with cellular respiration.
Check Your Understanding
• Where does the energy captured at the beginning of photosynthesis originate from?
• What is the form of chemical energy produced by photosynthesis?
• What occurs in oxidation and reduction reactions?
aerobic respiration Cellular respiration in the presence of oxygen.
alcoholic fermentation Fermentation in the absence of oxygen; produces ethyl alcohol (drinking alcohol) and
carbon dioxide; occurs in yeasts.
anaerobic respiration Cellular respiration in the absence of oxygen; fermentation.
ATP A usable form of energy inside the cell; adenosine triphosphate.
cellular respiration The process in which the energy in food is converted into energy that can be used by the
body’s cells; in other words, glucose is converted into ATP.
fermentation Anaerobic respiration in which NAD+ is recycled so that is can be reused in the glycolysis (the
breakdown of glucose) process.
lactic acid fermentation Anaerobic respiration that recycles NAD+ for glycolysis (the breakdown of glucose);
occurs in animals and some bacteria and fungi.
What is Cellular Respiration?
How does the food you eat provide energy? When you need a quick boost of energy, you might reach for an apple
or a candy bar. But cells do not "eat" apples or candy bars, these foods need to be broken down so that cells can use
them. Through the process of cellular respiration, the energy in food is changed into energy that can be used by
the body’s cells. In other words, glucose and oxygen are converted into ATP, carbon dioxide, and water. ATP, or
adenosine triphosphate, is chemical energy the cell can use. It is the molecule that provides energy for your cells to
perform work, such as moving your muscles as you walk down the street.
A video of cellular respiration can be seen at (7:58).
Click image to the left for more content.
The Process of Cellular Respiration
What happens inside of the cell? Glucose is broken down in the cytoplasm of the cells and then transported to the
mitochondria, the organelles known as the energy "powerhouses" of the cells (Figure 4.9 ). Inside the mitochondria,
the "broken-down" glucose is broken down again to release ATP. Oxygen is needed to help the process of turning
glucose into ATP. The initial step releases just two molecules of ATP for each glucose. The later steps release much
more ATP.
Most of the reactions of cellular respiration are carried out in the mitochondria.
The Reactants
What goes into the cell? Oxygen and glucose are both reactants in the process of cellular respiration. Oxygen enters
the body when an organism breathes. Glucose enters the body when an organism eats.
The Products
What does the cell produce? The main product of cellular respiration is ATP. Waste products include carbon dioxide
and water. Carbon dioxide is transported from your mitochondria out of your cell, to your red blood cells, and back
to your lungs to be exhaled.
When one molecule of glucose is broken down, it can be converted to a net total of 36 or 38 molecules of ATP. This
only occurs in the presence of oxygen.
The Chemical Reaction
The overall chemical reaction for cellular respiration is 1 molecule of glucose (C6 H12 O6 ) and 6 molecules of oxygen
(O2 ) yields 6 molecules of carbon dioxide (CO2 ) and 6 molecules of water (H2 0). Using chemical symbols the
equation is represented as follows:
C6 H12 O6 + 6O2 ! 6CO2 + 6H2 O
Connecting Cellular Respiration and Photosynthesis
Notice that the equation for cellular respiration is the direct opposite of photosynthesis (Figure 4.10 ). While water
was broken down to form oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen
to form water. While photosynthesis requires carbon dioxide and releases oxygen, cellular respiration requires
oxygen and releases carbon dioxide. This exchange of carbon dioxide and oxygen in all the organisms that use
photosynthesis or cellular respiration worldwide helps to keep atmospheric oxygen and carbon dioxide at stable
Sometimes cellular respiration is anaerobic, occurring in the absence of oxygen. In this process, called fermentation, no additional ATP is produced, so the organism only obtains the two ATP molecules per glucose molecule
from the initial step of this process (compare that to 36-38 ATP produced with oxygen!).
Yeasts (single-celled eukaryotic organisms) perform alcoholic fermentation in the absence of oxygen, making ethyl
alcohol (drinking alcohol) and carbon dioxide. This process is used to make common food and drinks. For example,
alcoholic fermentation is used to bake bread. The carbon dioxide bubbles allow the bread to rise, and the alcohol
evaporates. In wine making, the sugars of grapes are fermented to produce the wine.
Animals and some bacteria and fungi carry out lactic acid fermentation. Lactic acid is a waste product of this
process. Our muscles perform lactic acid fermentation during strenuous exercise, when oxygen cannot be delivered
to the muscles quickly enough. The buildup of lactic acid is what makes your muscles sore after exercise.
Bacteria that produce lactic acid are used to make cheese and yogurt (Figure 4.11 ). Tooth decay is also increased
by lactic acid from the bacteria that use the sugars in your mouth for energy.
Lesson Summary
• Cellular respiration is the breakdown of glucose to release energy in the form of ATP.
Cellular respiration and photosynthesis
are direct opposite reactions. Some of
the ATP made in the mitochondria is
used as energy for work and some is
lost to the environment as heat. Can you
explain what is depicted in this diagram
cheese lactic acid f ermentation and
wine alcoholic f ermentation.
• If oxygen is not available, the process of fermentation can break down glucose without the presence of oxygen.
A summary of cellular respiration can be viewed at (8:50).
Click image to the left for more content.
Review Questions
1. What is the purpose of cellular respiration?
2. Where is glucose broken down to form ATP?
Apply Concepts
3. What are the products of alcoholic fermentation?
4. Write the chemical reaction for the overall process of cellular respiration.
5. What produces more ATP, aerobic or anaerobic cellular respiration? What is the purpose of fermentation?
Critical Thinking
6. Why do your muscles get sore after vigorous exercise?
7. Why is the cellular respiration equation the opposite of the photosynthesis equation?
Supplemental Links
Points to Consider
• What do you think could happen if your cells divide uncontrollably?
• When new life is formed, do you think it receives all the DNA of the mother and the father?
• Why do you think you might need new cells throughout your life?
MS Cell Division,
Reproduction, and DNA
What has to happen for a cell to divide? Plenty. The above image shows the mitotic spindle in a sand dollar embryo.
The mitotic spindle separates DNA in cells that are dividing. But that is just one step in the process. Why do you
think cells need to divide? Do all cells divide the same way? How do cells help us reproduce? What would happen to
living things if their cells failed to divide? What happens if cells divide uncontrollably? Think about these questions
as you begin to understand why and how cells divide and how cell division helps the reproduction of all living things.
Cell Division
Lesson Objectives
• Explain why cells need to divide.
• List the stages of the cell cycle and explain what happens at each stage.
• List the stages of mitosis and explain what happens at each stage.
Check Your Understanding
• What is the cell theory?
• In what part of your cells is the genetic information located?
anaphase Third phase of mitosis and meiosis (anaphase I and anaphase II) where sister chromatids separate and
move to opposite sides of the cell.
cancer A disease in which abnormal cells divide out of control.
cell cycle Phases in the "life" of eukaryotic cells that leads to cell division.
chromosome DNA wound around proteins; forms during prophase of mitosis and meiosis.
cytokinesis Division of the cytoplasm after mitosis or meiosis.
daughter cell Cells that divide from the parent cell after mitosis or meiosis.
interphase Stage of the cell cycle when DNA is synthesized and the cell grows; composed of the first three phases
of the cell cycle.
metaphase Second phase of mitosis and meiosis (metaphase I and metaphase II) where the chromosomes are
aligned in the center of the cell.
mitosis Sequence of steps in which a nucleus is divided into two daughter nuclei, each with an identical set of
parent cell Cell that divides into daughter cells after mitosis or meiosis.
prophase Initial phase of mitosis and meiosis (prophase I and prophase II) where chromosomes condense, the
nuclear envelope dissolves and the spindle begins to form.
sister chromatids The two identical molecules of DNA in a chromosome after the DNA is replicated.
spindle Fibers that move chromosomes during mitosis and meiosis.
telophase Final phase of mitosis and meiosis (telophase I and telophase II) where a nuclear envelop forms around
each of the two sets of chromosomes.
Why Cells Divide
Imagine the first stages of life. In humans, a sperm fertilizes an egg, forming the first cell. But humans are made up
of trillions of cells, so where do the new cells come from? Remember that according to cell theory, all cells must
come from existing cells. From that one cell, an entire baby will develop.
How does a new life go from one cell to so many? The cell divides in half, creating two cells. Then those two cells
divide, for a total of four cells. The new cells continue to divide and divide. One cell becomes two, then four, then
eight, and so on (Figure 5.1 ).
Cells divide repeatedly to produce an
embryo. Previously the one-celled zygote the f irst cell o f a new organism
divided to make two cells a. Each of
the two cells divides to yield four cells b
then the four cells divide to make eight
cells c and so on. Through cell division
an entire embryo forms from one initial
Besides the development of a baby, there are many other reasons that cell division is necessary for life:
a. To grow and develop, you must form new cells. Imagine how often your cells must divide during a growth
spurt. Growing just an inch requires countless cell divisions.
b. Cell division is also necessary to repair damaged cells. Imagine you cut your finger. After the scab forms, it
will eventually disappear and new skin cells will grow to repair the wound. Where do these cells come from?
Some of your existing skin cells divide and produce new cells.
c. Your cells can also simply wear out. Over time you must replace old and worn-out cells. Cell division is
essential to this process.
The Cell Cycle
The process of cell division in eukaryotic cells is carefully controlled. The cell cycle is the lifecycle of a cell, with
cell division at the end of the cycle. Like a human lifecycle that is made up of different phases, like childhood,
adolescence, and adulthood, there are a series of steps that lead to cell division (Figure 5.2 ).
These steps can be divided into two main components, interphase and mitosis.
a. Interphase: The stage when the cell mostly performs its “everyday” functions. For example, it is when a
kidney cell does what a kidney cell is supposed to do.
b. Mitosis: The stage when the cell prepares to become two cells.
Most of the cell cycle consists of interphase, the time between cell divisions. Interphase can be divided into three
a. The first growth phase (G1): During the G1 stage, the cell doubles in size and doubles the number of organelles.
b. The synthesis phase (S): The DNA is replicated during this phase. In other words, an identical copy of all
the cell’s DNA is made. This ensures that each new cell has a set of genetic material identical to that of the
parental cell. DNA replication will be further discussed in lesson 5.3.
c. The second growth phase (G2): Proteins are synthesized that will help the cell divide. At the end of interphase,
the cell is ready to enter mitosis.
The cell cycle is the repeated process
of growth and division. Notice that most
of the cell cycle is spent in interphase
G1 S and G2 I . G0 is a resting state of
the cell cycle.
During mitosis, the nucleus divides. Mitosis is followed by cytokinesis, when the cytoplasm divides, resulting in
two cells. After cytokinesis, cell division is complete. Scientists say that one parent cell, or the dividing cell, forms
two genetically identical daughter cells, or the cells that divide from the parent cell. The term "genetically identical"
means that each cell has an identical set of DNA, and this DNA is also identical to that of the parent cell. If the cell
cycle is not carefully controlled, it can cause a disease called cancer, which causes cell division to happen too fast.
A tumor can result from this kind of growth.
Cancer is discussed in the video at
L7LDPk8w. (12:36).
Click image to the left for more content.
Two animations of the cell cycle are available at the following links. See if you can explain what is happening in
these animations.
Mitosis and Chromosomes
The genetic information of the cell, or DNA, is stored in the nucleus. During mitosis, two nuclei (plural for nucleus)
must form, so that one nucleus can be in each of the new cells. The DNA inside of the nucleus is also copied. The
copied DNA needs to be moved into the nucleus, so each cell can have a correct set of genetic instructions.
To begin mitosis, the DNA in the nucleus wraps around proteins to form chromosomes. Each organism has a unique
number of chromosomes. In human cells, our DNA is divided up into 23 pairs of chromosomes. After the DNA
is replicated during the S stage of interphase, each chromosome has two identical molecules of DNA, called sister
chromatids, forming the "X" shaped molecule depicted in Figure 5.3 .
The DNA double helix wraps around
proteins 2 and tightly coils a number of
times to form a chromosome 5. This figure shows the complexity of the coiling
process. The red dot shows the location of the centromere where the microtubules attach during mitosis and meiosis.
The Four Phases of Mitosis
During mitosis, the two sister chromatids must be split apart. Each resulting chromosome is made of 1/2 of the "X".
Through this process, each daughter cell receives one copy of each chromosome. Mitosis is divided into four phases
(Figure 5.4 ):
a. Prophase: The chromosomes "condense," or become so tightly wound that you can see them under a microscope. The wall around the nucleus, called the nuclear envelope, disappears. Spindles also form and attach to
chromosomes to help them move.
b. Metaphase: The chromosomes line up in the center of the cell. The chromosomes line up in a row, one on
top of the next.
c. Anaphase: The two sister chromatids of each chromosome separate, resulting in two sets of identical chromosomes.
d. Telophase: The spindle dissolves and nuclear envelopes form around the chromosomes in both cells.
An overview of the cell cycle and mitosis during prophase the chromosomes
condense during metaphase the chromosomes line up during anaphase the
sister chromatids are pulled to opposite
sides of the cell and during telophase
the nuclear envelope forms.
Each new nucleus contains the exact same number and type of chromosomes as the original cell. The cell is now
ready for cytokinesis, which literally means "cell movement." The cells separate, producing two genetically identical
cells, each with its own nucleus. Figure 5.5 is a representation of dividing plant cells.
The phases of mitosis are discussed in the video:
0CF64/8/LLKX_4DHE3I (20:42).
Click image to the left for more content.
Additional animations of mitosis can be viewed at the following links:
This is a representation of dividing plant
cells. Cell division in plant cells differs
slightly from animal cells as a cell wall
must form. Note that most of the cells
are in interphase. Can you find examples of the different stages of mitosis
Mitosis in Real Time can be viewed at;feature=related (0:19).
Click image to the left for more content.
Lesson Summary
Cells divide for growth, development, reproduction and replacement of injured or worn-out cells.
The cell cycle is a series of controlled steps by which a cell divides.
During mitosis, the newly duplicated chromosomes are divided into two daughter nuclei.
This summary diagram depicts one cell dividing into two genetically identical cells. Mitosis occurs after DNA
replication. A diploid cell has two sets of chromosomes, as is shown here.
Review Questions
1. In what phase of mitosis are chromosomes moving toward opposite sides of the cell?
2. In what phase of mitosis do the duplicated chromosomes condense?
3. What step of the cell cycle is the longest?
4. What is the term for the division of the cytoplasm?
5. What happens during the S stage of interphase?
Apply Concepts
6. Interphase used to be considered the “resting” stage of the cell cycle. Why is this not correct?
7. What are some reasons that cells divide?
8. During what stage of the cell cycle does the cell double in size?
9. Why must cell division be tightly regulated?
Critical Thinking
10. What would happen if the cells in your liver stopped going through the process of mitosis?
11. What do you think might happen if mitosis could NOT stop happening to the cells in your brain?
Further Reading / Supplemental Links
Points to Consider
• How might a cell without a nucleus divide?
• How are new cells made that include the DNA of two parents?
Lesson Objectives
• Name the types of asexual reproduction.
• Explain the advantage of sexual reproduction.
• List the stages of meiosis and explain what happens in each stage.
Check Your Understanding
• Can something that does not reproduce still be considered living?
• What stores the genetic information that is passed on to offspring?
• How many chromosomes are in the human nucleus?
allele An alternative form of a gene.
asexual reproduction A form of reproduction in which a new individual is created by only one parent.
binary fission An asexual form of reproduction where a cell splits into two daughter cells.
crossing-over Exchange of DNA segments between homologous chromosomes; occurs during prophase I of meiosis.
cross-pollination Sexual reproduction in plants where sperm from the pollen of one flower is received by the ovary
of another flower.
diploid When a cell has two sets of chromosomes.
external fertilization Reproduction where the eggs are fertilized outside the body.
gamete Haploid sex cell; egg or sperm.
gonad Organ that produces gametes, such as the ovaries and testes.
haploid When a cell has only one set of chromosomes, typical of a gamete.
internal fertilization Reproduction that occurs through the internal deposit of gametes.
meiosis Nuclear division that results in haploid gametes.
ovaries Female reproductive organs that produce eggs and secrete estrogen.
parthenogenesis Reproduction where an unfertilized egg develops into a new individual.
sexual reproduction Reproduction where gametes from two parents combine to make an individual with an
unique set of genes.
testes Male reproductive organs that produce sperm and secrete testosterone.
zygote Cell that forms when a sperm and egg unite; the first cell of a new organism.
What is reproduction?
What does reproduction mean? Can an organism be considered alive if it cannot make the next generation? Since
individuals cannot live forever, they must reproduce for the species to survive. Reproduction is the ability to make
the next generation.
Two methods of reproduction are:
a. Asexual reproduction, or the process of forming a new individual from a single parent.
b. Sexual reproduction, or the process of forming a new individual from two parents.
There are advantages and disadvantages to each method, but the result is always the same: a new life begins.
Asexual Reproduction
For humans to reproduce, DNA must be passed from the mother and father to the child. Humans cannot reproduce
with just one parent, but it is possible in other organisms, like bacteria, some insects and some fish. These organisms
can reproduce asexually, meaning that the offspring (children) have a single parent and share the exact same genetic
material as the parent. This is very different from humans.
The advantage of asexual reproduction is that it can be very quick and does not require the meeting of a male and
female organism. The disadvantage of asexual reproduction is that organisms cannot mix beneficial traits from both
parents. An organism that is born through asexual reproduction only has the DNA from the one parent, and it is the
exact copy of that parent. This can cause problems for the individual. For example, if the parent organism has a
gene that causes cancer, the offspring will also have the gene that causes cancer. Organisms produced sexually may
or may not inherit the cancerous gene because there are two parents mixing up their genes.
Types of organisms that reproduce asexually include:
a. Prokaryotic organisms, like bacteria. Bacteria reproduce through binary fission, where they grow and divide
in half (Figure 5.6 ). First, their chromosome replicates (bacteria only have one chromosome) and the cell
enlarges. After cell division, the two new cells each have one identical chromosome (mitosis is not necessary
because bacteria do not have nuclei). Then, new membranes form to separate the two cells. This simple
process allows bacteria to reproduce very rapidly.
b. Flatworms, an animal species. Flatworms divide in two, then each half regenerates into a new flatworm
identical to the original.
c. Different types of insects, fish, and lizards. These organisms can reproduce asexually through a process called
parthenogenesis (Figure 5.7 ). Parthenogenesis happens when an unfertilized egg cell grows into a new
organism. The resulting organism has half the amount of genetic material of the parent. Parthenogenesis is
common in honeybees. In a hive, the sexually produced eggs become workers, while the asexually produced
eggs become drones.
Bacteria reproduce by binary fission.
Shown is one bacterium reproducing
and becoming two bacteria.
Sexual Reproduction
During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a separate male
and female sex, with the male producing sperm and the female producing eggs. When a sperm and egg meet, a
zygote, the first cell of a new organism, is formed (Figure 5.8 ). The zygote will divide and grow into the embryo.
Let’s explore how animals, plants, and fungi reproduce sexually:
• Animals often have gonads, organs that produce eggs or sperm. The male gonads are the testes, which produce
the sperm, and the female gonads are the ovaries, which produce the eggs. Sperm and egg, the two sex cells,
are known as gametes, and can combine two different ways:
This Komodo dragon was born by
During sexual reproduction a sperm fertilizes an egg.
a. Fish and other aquatic animals release their gametes in the water, which is called external fertilization. These
gametes will combine by chance. (Figure 5.9 ).
b. Animals that live on land reproduce by internal fertilization. Typically males have a penis that deposits
sperm into the vagina of the female. Birds do not have penises, but they do have a chamber called the cloaca
that they place close to another bird’s cloaca to deposit sperm.
This fish guards her eggs which will be
fertilized externally.
• Plants can also reproduce sexually, but their reproductive organs are different from animals’ gonads. Plants
that have flowers have their reproductive parts in the flower. The sperm is contained in the pollen, while the
egg is contained in the ovary, deep within the flower. The sperm can reach the egg two different ways:
a. In self-pollination, the egg is fertilized by the pollen of the same flower.
b. In cross-pollination, sperm from the pollen of one flower fertilizes the egg of another flower. Like other types
of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination occurs when
pollen is carried by the wind to another flower. It can also occur when animal pollinators, like honeybees, or
butterflies (Figure 5.10 ) carry the pollen from flower to flower.
• Fungi can also reproduce sexually, but instead of female and male sexes, they have (+) and (-) strains. When
the filaments of a (+) and (-) fungi meet, the zygote is formed. Just like in plants and animals, each zygote
receives DNA from two parent strains.
Meiosis and Gametes
Meiosis is a process of cell division that produces sex cells, or gametes. Gametes are reproductive cells, such as
sperm and egg. As gametes are produced, the number of chromosomes must be reduced by half. Why? The zygote
must contain information from the mother and from the father, so the gametes must contain half of the chromosomes
found in normal body cells.
Butterflies receive nectar when they deposit pollen into flowers resulting in
In humans, our cells have 23 pairs of chromosomes, and each chromosome within a pair is called a homologous
chromosome. For each of the 23 chromosome pairs, you received one chromosome from your father and one
chromosome from your mother. The homologous chromosomes are separated when gametes are formed. Therefore,
gametes have only 23 chromosomes, not 23 pairs.
Alleles are alternate forms of genes found on chromosomes. Since the separation of chromosomes into gametes
is random, it results in different combinations of chromosomes (and alleles) in each gamete. With 23 pairs of
chromosomes, there is a possibility of over 8 million different combinations of chromosomes in a gamete.
Haploid vs. Diploid
A cell with two sets of chromosomes is diploid, referred to as 2n, where n is the number of sets of chromosomes.
Most of the cells in a human body are diploid. A cell with one set of chromosomes, such as a gamete, is haploid,
referred to as n. Sex cells are haploid. When a haploid sperm (n) and a haploid egg (n) combine, a diploid zygote
will be formed (2n). In short, when a diploid zygote is formed, half of the DNA comes from each parent.
Before meiosis begins, DNA replication occurs, so each chromosome contains two sister chromatids that are identical
to the original chromosome.
Meiosis is divided into two divisions: Meiosis I and Meiosis II. Each division is similar to mitosis and can be divided
into the same phases: prophase, metaphase, anaphase, and telophase. Between the two divisions, DNA replication
does not occur. Through this process, one diploid cell will divide into four haploid cells.
The phases of meiosis are discussed at
ijLc52LmFQg (27:23).
Click image to the left for more content.
Meiosis I
During meiosis I, the pairs of homologous chromosomes are separated from each other.
a. Prophase I: The homologous chromosomes line up together. During this time, a process that only happens
in meiosis can occur. This process is called crossing-over (Figure 5.11 ), which is the exchange of DNA
between homologous chromosomes. Crossing-over increases the new combinations of alleles in the gametes.
Without crossing-over, the offspring would always inherit all of the many alleles on one of the homologous
chromosomes. Also during prophase I, the spindle forms, the chromosomes condense as they coil up tightly,
and the nuclear envelope disappears.
b. Metaphase I: The homologous chromosomes line up in pairs in the middle of the cell. Chromosomes from the
mother or from the father can each attach to either side of the spindle. Their attachment is random, so all of
the chromosomes from the mother or father do not end up in the same gamete. The gamete will contain some
chromosomes from the mother and some chromosomes from the father.
c. Anaphase I: The homologous chromosomes separate.
d. Telophase I: The spindle fibers dissolves, but a new nuclear envelope does not need to form. This is because
the nucleus will divide again. No DNA replication happens between meiosis I and meiosis II because the
chromosomes are already duplicated.
During crossing-over segments of DNA
are exchanged between sister chromatids. Notice how this can result in an
allele A on one sister chromatid being
moved onto the other sister chromatid.
Meiosis II
During meiosis II, the sister chromatids are separated and the gametes are generated.
The steps are outlined below:
Prophase II: The chromosomes condense.
Metaphase II: The chromosomes line up one on top of the next along the middle of the cell.
Anaphase II: The sister chromatids separate.
Telophase II: Nuclear envelopes form around the chromosomes in all four cells.
After cytokinesis, each cell has divided again. Therefore, meiosis results in four daughter cells with half the DNA of
the parent cell (Figure 5.12 ). In human cells, the parent cell has 46 chromosomes, so the cells produced by meiosis
have 23 chromosomes. These cells will become gametes.
An overview of meiosis.
Mitosis vs. Meiosis: A Comparison
Mitosis, meiosis and sexual reproduction are discussed at
C5110CF64/7/kaSIjIzAtYA (18:23).
Click image to the left for more content.
Figure 5.13 is a comparison between binary fission, mitosis, and meiosis. Mitosis and meiosis are also compared in
Table 5.1 .
A comparison between binary fission
mitosis and meiosis.
Animations of meiosis can be found at the following sites:
TABLE 5.1:
Number of cells produced:
Rounds of Cell Division:
Haploid or Diploid:
Daughter cells identical to parent
Daughter cells identical to each
To produce new cells
To produce gametes
Lesson Summary
• Organisms can reproduce sexually or asexually.
• The gametes in sexual reproduction must have half the DNA of the parent.
• Meiosis is the process of nuclear division that forms gametes.
Review Questions
1. What is parthogenesis?
2. During what phase of meiosis do homologous chromosomes separate?
3. What is the purpose of meiosis?
4. In what phase of meiosis do homologous chromosomes pair up?
Apply Concepts
5. Explain how organisms reproduce asexually.
6. Explain how birds fertilize their eggs.
7. How do most plants reproduce sexually?
8. Compare and contrast the process of mitosis and the process of meiosis.
Critical Thinking
9. How would sexual reproduction in a lizard be different than in a fish?
10. What is the advantage of sexual reproduction over asexual reproduction?
11. If an organism has 12 chromosomes in its cells, how many chromosomes will be in its gametes?
Further Reading / Supplemental Links
Points to Consider
• What must be replicated prior to mitosis?
• How do you think DNA might be replicated?
• What might happen if there is a mistake during DNA replication?
DNA, RNA, and Protein Synthesis
Lesson Objectives
Explain the chemical composition of DNA.
Explain how DNA synthesis works.
Explain how proteins are coded for and synthesized.
Describe the three types of RNA and the functions of each.
Check Your Understanding
• What is the purpose of DNA?
• When is DNA replicated?
amino acid The units that combine to make proteins.
DNA Deoxyribonucleic acid; a nucleic acid that is the genetic material of all organisms.
DNA replication The synthesis of new DNA; occurs during the S phase of the cell cycle.
double helix The shape of DNA; a double spiral, similar to a spiral staircase.
gene The inherited unit of DNA that encodes for one protein (or one polypeptide).
mutagen A chemical or physical agent that can cause changes to accumulate in DNA.
mutation A change in the nucleotide sequence of DNA.
nucleotide The units that make up DNA; consists of a 5-carbon sugar, a phosphate group, and a nitrogen-containing
RNA The nucleic acid that carries the information stored in DNA to the ribosome.
semiconservative replication Describes how the replication of DNA results in two molecules of DNA, each with
one original strand and one new strand.
transcription The synthesis of a RNA that carries the information encoded in the DNA.
translation The synthesis of proteins as the ribosome reads each codon in RNA, which code for a specific amino
What is DNA?
DNA, is the material that makes up our chromosomes and stores our genetic information. When you build a house,
you need a blueprint, a set of instructions that tells you how to build. The DNA is like the blueprint for living
organisms. The genetic information is a set of instructions that tell your cells what to do.
DNA is an abbreviation for deoxyribonucleic acid. As you may recall, nucleic acids are a type of macromolecule
that store information. The deoxyribo part of the name refers to the name of the sugar that is contained in DNA,
deoxyribose. DNA may provide the instructions to make up all living things, but it is actually a very simple molecule.
DNA is made of a long chain of nucleotides.
Nucleotides are composed of 3 main parts:
a. Phosphate group
b. 5-carbon sugar
c. Nitrogen-containing base
The only difference between each nucleotide is the identity of the base. There are only four possible bases that make
up each DNA nucleotide: adenine (A), guanine (G), thymine (T), and cytosine (C).
The various sequences of these four bases make up the genetic code of your cells. It may seem strange that there are
only four letters in the “alphabet” of DNA. But since your chromosomes contain millions of nucleotides, there are
many, many different combinations possible with those four letters.
But how do all these pieces fit together? James Watson and Francis Crick won the Nobel Prize in 1962 for piecing
together the structure of DNA. Together with the work of Rosalind Franklin and Maurice Wilkins, they determined
that DNA is made of two strands of nucleotides formed into a double helix, or a two-stranded spiral, with the sugar
and phosphate groups on the outside, and the paired bases connecting the two strands on the inside of the helix
(Figure 5.14 and Figure 5.15 ).
The bases in DNA do not pair randomly. When Erwin Chargaff looked closely at the bases in DNA, he noticed
that the percentage of adenine (A) in the DNA always equaled the percentage of thymine (T), and the percentage
of guanine (G) always equaled the percentage of cytosine (C). Watson and Crick’s model explained this result by
suggesting that A always pairs with T and G always pairs with C in the DNA helix. Therefore A and T, and G and C,
are "complementary bases," or bases that always pair together. For example, if one DNA strand reads ATGCCAGT,
the other strand will be made up of the complementary bases: TACGGTCA.
The vocabulary of DNA, including chromosomes, chromatids, chromatin, transcription, translation, and replication,
is discussed at (18:23).
Click image to the left for more content.
DNA&#8217 s three-dimensional structure is a double helix. The hydrogen
bonds between the bases at the center
of the helix hold the helix together.
The chemical structure of DNA includes
a chain of nucleotides consisting of a 5carbon sugar a phosphate group and
a nitrogen base. Notice how the sugar
and phosphate form the backbone of
DNA one strand in blue with the hydrogen bonds between the bases joining the two strands.
DNA Replication
The base pairing rules are crucial for the process of replication. DNA replication occurs when DNA is copied to
form an identical molecule of DNA. DNA replication happens before cell division. Below are the steps involved in
DNA replication:
a. The DNA helix unwinds like a zipper, as the bonds between the base pairs are broken.
b. The two single strands of DNA then each serve as a template for a new stand to be created. Using DNA as a
template means that the bases are placed in the right order because of the base pairing rules. If ATG is on the
"template strand," then TAC will be on the new DNA strand.
c. The new set of nucleotides then join together to form a new strand of DNA. The process results in two DNA
molecules, each with one old strand and one new strand of DNA.
This process is known as semiconservative replication because one strand is conserved (kept the same) in each new
DNA molecule (Figure 5.16 ).
DNA replication occurs when the DNA
strands &#8220 unzip&#8221 and the
original strands of DNA serve as a template for new nucleotides to join and
form a new strand.
Protein Synthesis
The DNA sequence contains the instructions to make units called amino acids, which are assembled in a specific
order to make proteins. In short, DNA contains the instructions to create proteins. Each strand of DNA has many
separate sequences that code for a specific protein. Units of DNA that contain code for the creation of one protein
are called genes. An overview of protein synthesis can be seen at this animation:
Cells Can Turn Genes On or Off
There are about 22,000 genes in every human cell. Does every human cell have the same genes? Yes. Does every
human cell use the same genes to make the same proteins? No. In a multicellular organism, such as us, cells have
specific functions because they have different proteins. They have different proteins because different genes are
expressed in different cell types.
Imagine that all of your genes are "turned off." Each cell type only "turns on" (or expresses) the genes that have the
code for the proteins it needs to use. So different cell types "turn on" different genes, allowing different proteins to
be made, giving different cell types different functions.
Three Types of RNA
DNA contains the instructions to create proteins, but it does not make proteins itself. DNA is located in the nucleus,
while proteins are made on ribosomes in the cytoplasm. So DNA needs a messenger to bring its instructions to
a ribosome located outside of the nucleus. DNA sends out a message, in the form of RNA (ribonucleic acid),
describing how to make the protein.
There are three types of RNA directly involved in protein synthesis:
• Messenger RNA (mRNA) carries the instructions from the nucleus to the cytoplasm.
• The other two forms of RNA, ribosomal RNA (rRNA) and transfer RNA (tRNA) are involved in the process
of ordering the amino acids to make the protein.
All three RNAs are nucleic acids, made of nucleotides, similar to DNA. The RNA nucleotide is different from the
DNA nucleotide in the following ways:
• RNA contains a different kind of sugar, called ribose.
• In RNA, the base uracil (U) replaces the thymine (T) found in DNA.
• RNA is a single strand.
DNA Transcription
mRNA is created by using DNA as a template. The process of constructing an mRNA molecule from DNA is known
as transcription (Figure 5.17 and Figure 5.18 ). The double helix of DNA unwinds and the nucleotides follow
basically the same base pairing rules to form the correct sequence in the mRNA. This time, however, U pairs with
each A in the DNA. In this manner, the genetic code is passed on to the mRNA.
Two multimedia links of protein synthesis are provided below.
Transcription and Translation can be viewed at (4:06).
Click image to the left for more content.
Each gene a contains triplets of bases b
that are transcribed into RNA c. Every
triplet or codon encodes for a unique
amino acid.
Base-pairing ensures the accuracy of
transcription. Notice how the helix must
unwind for transcription to take place.
RNA Translation
The mRNA is directly involved in the protein-making process. mRNA tells the ribosome (Figure 5.19 ) how to
create a protein. The process of reading the mRNA code in the ribosome to make a protein is called translation
(Figure 5.20 ). Sets of three bases, called codons, are read in the ribosome, the organelle responsible for making
The following are the steps involved in translation:
a. mRNA travels to the ribosome from the nucleus.
b. The base code in the mRNA determines the order of the amino acids in the protein. The genetic code in mRNA
is read in “words” of three letters (triplets), called codons. There are 20 amino acids and different codons code
for different ones. For example, GGU codes for the amino acid glycine, while GUC codes for valine.
c. tRNA reads the mRNA code and brings a specific amino acid to attach to the growing chain of amino acids.
Each tRNA carries only one type of amino acid and only recognizes one specific codon.
d. tRNA is released from the amino acid.
e. Three codons, UGA, UAA, and UAG, indicate that the protein should stop adding amino acids. They are
called "stop codons" and do not code for an amino acid. Once tRNA comes to a stop codon, the protein is set
free from the ribosome.
The chart in Figure 5.21 is used to determine which amino acids correspond to which codons. An interactive activity
for transcribing and translating a gene can be found at
Ribosomes translate RNA into a protein
with a specific amino acid sequence.
The tRNA binds and brings to the ribosome the amino acid encoded by the
mRNA. Ribosomes are made of rRNA
and proteins.
This summary of how genes are expressed shows that DNA is transcribed
into RNA which is translated in turn to
The process of DNA replication is not always 100% accurate, and sometimes the wrong base is inserted in the new
strand of DNA. A permanent change in the sequence of DNA is known as a mutation. Small changes in the DNA
sequence are usually point mutations, which is a change in a single nucleotide. A mutation may have no effect.
This chart shows the genetic code used
by all organisms. For example an RNA
codon reading GUU would encode for a
valine Val according to this chart. Start
at the center for the first base of the
three base codon and work your way
out. Notice for valine the second base
is a U and the third base of the codon
may be either a G C A or U. Similarly
glycine Gly is encoded by a GGG GGA
GGC and GGU.
Sometimes, a mutation can cause the protein to be made incorrectly, which can affect how well the protein works,
or whether it works at all. Usually the loss of a protein function is detrimental to the organism.
However, in rare circumstances, the mutation can be beneficial. For example, suppose a mutation in an animal’s
DNA causes the loss of an enzyme that makes a dark pigment in the animal’s skin. If the population of animals has
moved to a light colored environment, the animals with the mutant gene would have a lighter skin color and be better
camouflaged. So in this case, the mutation is beneficial.
Mutations may also occur in chromosomes. Possible types of mutations in chromosomes (Figure 5.22 ) include:
Deletion: When a segment of DNA is lost, so there is a missing segment in the chromosome.
Duplication: When a segment of DNA is repeated, creating a longer chromosome.
Inversion: When a segment of DNA is flipped and then reattached to the chromosome.
Insertion: When a segment of DNA from one chromosome is added to another, unrelated chromosome.
Translocation: When two segments from different chromosomes change positions.
If a single base is deleted (called a point mutation), there can be huge effects on the organism because this may cause
a "frameshift mutation." Remember that the bases are read in groups of three by the tRNA. If the reading frame gets
off by one base, the resulting sequence will consist of an entirely different set of codons. The reading of an mRNA
is like reading three letter words of a sentence. Imagine you wrote “the big dog ate the red cat”. If you take out
the second letter from "big", the frame will be shifted so now it will read “ the bgd oga tet her edc at.” One single
deletion makes the whole “sentence” impossible to read.
Many mutations are not caused by errors in replication. Mutations can happen spontaneously and they can be
caused by mutagens in the environment. Some chemicals, such as those found in tobacco smoke, can be mutagens.
Sometimes mutagens can also cause cancer. Tobacco smoke, for example, is often linked to lung cancer.
Mutations can arise in DNA through
deletion duplication inversion insertion
and translocation within the chromosome.
Lesson Summary
• DNA stores the genetic information of the cell in the sequence of its 4 bases: adenine, thymine, guanine, and
• The information in a small segment of DNA, a gene, is sent by mRNA to the ribosome to synthesize a protein.
• Within the ribosome, tRNA reads the mRNA in sets of three bases (triplets), called codons, which encode for
the specific amino acids that make up the protein.
• A mutation is a permanent change in the sequence of bases in DNA.
Review Questions
1. What is a nucleotide made out of?
2. Describe the process of DNA replication.
3. What is made in the process of transcription?
4. What is made in the process of translation?
5. Name a mutagen.
Apply Concepts
6. Translate the following segment of DNA into RNA: AGTTC
7. Write the complimentary DNA nucleotides to this strand of DNA: GGTCCA
8. Nucleotides are subunits of which two macromolecules?
9. Amino acids are subunits that make up what macromolecule?
10. How does RNA encode for proteins?
Critical Thinking
11. How does a mutation in a strand of DNA affect translation and transcription?
12. Given the DNA sequence, ATGTTAGCCTTA, what is the mRNA sequence? What is the amino acid sequence?
Further Reading / Supplemental Links
Points to Consider
• Your cells have “proofreaders” that replace mismatched pairs that occurred during DNA synthesis. How would
that affect the rate of mutation in your body?
• There are many diseases due to mutations in the DNA. These are known as genetic diseases, and many can be
passed onto the next generation. Think about how a single base change cause a huge medical problem.
• Your DNA contains the instructions to make you. So is everyone’s DNA different? Can it be used to distinguish individuals, like a fingerprint?
MS Genetics
The above puppies are offspring from the same parents. Why don’t the puppies look identical to each other? Do you
think they are identical to their parents? Or do you think they have some traits from their mother and some from
their father?
Just as you don’t look exactly like your parents, neither do these puppies. Has anyone ever told you, “You have your
father’s eyes”? or, “You have red hair like your grandmother; it must have skipped a generation.” You don’t look
identical to your parents or to your grandparents. But many of their traits are passed down to you.
Genetics seeks to answer why and how we, and all living things, inherit traits from the previous generation.
Gregor Mendel and the Foundations of
Lesson Objectives
• Explain Mendel’s law of segregation.
• Draw a Punnett square to make predictions about the traits of the offspring of a simple genetic cross.
Check Your Understanding
• What is the genetic material of our cells?
• How does meiosis affect the chromosome number in gametes?
dominant Masks the expression of the recessive trait.
F1 generation The first filial generation; offspring of the P or parental generation.
F2 generation The second filial generation; offspring from the self-pollination of the F1 generation.
genetics The study of inheritance.
Punnett square Visual representation of a genetic cross that helps predict the expected ratios in the offspring, first
described by Reginald C. Punnett in the early 20th century.
recessive Expression is masked by the dominant factor (allele); only expressed if both factors are recessive.
Mendel’s Experiments
What does the word "inherit" mean? You may have inherited something of value from a grandparent or another
family member. To inherit is to receive something from someone who came before you. You can inherit objects, but
you can also inherit traits. If you inherit a trait from your parents, you can be inheriting their eye color, hair color,
or even the shape of your nose and ears!
Genetics is the study of inheritance. The field of genetics seeks to explain how traits are passed on from one
generation to the next.
An introduction to heredity can be seen at
4/12/eEUvRrhmcxM (17:27).
Click image to the left for more content.
In the late 1850s, an Austrian monk named Gregor Mendel (Figure 6.1 ) performed the first genetics experiments.
To study genetics, Mendel chose to work with pea plants because they have easily identifiable traits (Figure 6.2 ).
For example, pea plants are either tall or short, which are easily identifiable traits. Pea plants grow quickly, so he
could complete many experiments in a short period of time.
Gregor Mendel
Mendel also used pea plants because they can either self-pollinate or be cross-pollinated by hand, by moving pollen
from one flower to the stigma of another. When one plant’s sex cells combine with another plant’s sex cells, it is
called a "cross." These crosses produce offspring (or children), just like when male and female animals mate. Since
Mendel could move pollen between plants, he could carefully observe the results of crosses between two different
types of plants.
He studied the inheritance patterns for many different traits in peas, including round seeds versus wrinkled seeds,
white flowers versus purple flowers, and tall plants versus short plants. Because of his work, Mendel is considered
the "Father of Genetics."
Mendel’s First Experiment
In one of Mendel’s early experiments, he crossed a short plant and a tall plant. What do you predict the offspring
(children) of these plants were? Medium-sized plants? Most people during Mendel’s time would have said mediumsized. But an unexpected result occurred.
Characteristics of pea plants.
Mendel observed that the offspring of this cross (called the F1 generation) were all tall plants!
Next, Mendel let the F1 generation self-pollinate. That means the tall plant offspring were crossed with each other.
He found that 75% of their offspring (the F2 generation) were tall, while 25% were short. Shortness skipped a
generation. But why? In all, Mendel studied seven characteristics, with almost 20,000 F2 plants analyzed. All of his
results were similar to the first experiment - about three out of every four plants had one trait, while just one out of
every four plants had the other.
For example, he crossed purple flowered-plants and white flowered-plants. Do you think the colors blended? No,
they did not. Just like the previous experiment, all offspring in this cross (the F1 generation) were one color: purple.
In the F2 generation, 75% of plants had purple flowers and 25% had white flowers. There was no blending of traits
in any of Mendel’s experiments.
An interactive pea plant experiment can be found at nbs/projects/anthro201/exper/.
Mendel had to come up with a theory of inheritance to explain his results. He developed a theory called "the law of
He proposed that each pea plant had two hereditary factors for each trait. There were two possibilities for each
hereditary factor, such as short or tall. One factor is dominant to the other. The other trait that is masked is
called the recessive factor, meaning that when both factors are present, only the effects of the dominant factor are
Each parent can only pass on one of these factors to the offspring. When the sex cells, or gametes (sperm or egg),
form, the heredity factors must separate, so there is only one factor per gamete. In other words, the factors are
"segregated" in each gamete. When fertilization occurs, the offspring receive one factor from each gamete, so the
offspring have two hereditary factors.
This law explains what Mendel had seen in the F1 generation, because the two heredity factors were the short and
tall factors. Each individual in the F1 would have one of each factor, and as the tall factor is dominant to the short
factor, all the plants appeared tall.
In the F2 generation, produced by the cross of the F1, 25% of the offspring would have two short heredity factors,
so they would appear short. 75% would have at least one tall heredity factor and would be tall (see below).
In genetics problems the dominant factor is represented with a capital letter (T) while the recessive factor is repreCHAPTER 6. MS GENETICS
sented by a lowercase letter (t). For the T and t factors, three combinations are possible: TT, Tt, and tt. TT plants
will be tall, while plants with tt will be short. Since T is dominant to t, plants that are Tt will be tall because the
dominant factor masks the recessive factor.
Tall x Short
The above example crosses a TT tall plant with a tt short plant. As each parent gives one factor to the F1 generation,
all of the F1 generation will be Tt tall plants. When the F1 are allowed to self-pollinate, each parent will give one
factor to the F2 generation. The F2 offspring will be TT, Tt, tT or tt. That is, 75% (3 of 4) tall and 25% (1 of 4)
short. A Punnett Square helps determine these possibilities.
Probability and Punnett Squares
If this is confusing, don’t worry. A Punnett Square is a special tool used to predict the offspring from a cross, or
mating between two parents.
An explanation of Punnett squares can be viewed at
0CF64/13/D5ymMYcLtv0 (25:16).
Click image to the left for more content.
An example of a Punnett square (Figure 6.3 ) shows the results of a cross between two purple flowers that each have
one dominant factor and one recessive factor (Bb).
To create a Punnett Square, perform the following steps:
Take the factors from the first parent and place them at the top of the square (B and b)
Take the factors from the second parent and line them up on the left side of the square (B and b).
Pull the factors from the top into the boxes below.
Pull the factors from the side into the boxes next to them.
The possible offspring are represented by the letters in the boxes, with one factor coming from each parent.
Top left box: BB, or Purple flowers
Top right box: Bb, or Purple flowers
Lower left box: Bb, or Purple flowers
Lower right box: bb, or White flowers
Only one of the plants out of the four, or 25% of the plants, has white flowers (bb). The other 75% have purple
flowers (BB, Bb) because the color purple is the dominant trait in pea plants.
Now imagine you cross one of the white flowers (bb) with a purple flower that has both a dominant and recessive
factor (Bb). The only possible gamete in the white flower is the recessive (b), while the purple flower can have
gametes with either dominant (B) or recessive (b).
The Punnett Square of a cross between
two purple flowers Bb
Practice using a Punnett square with this cross (see Table ?? ).
Did you find that 50% of the offspring will be purple and 50% of the offspring will be white?
Practice more Punnett Squares here:
TABLE 6.1: The Punnett Square defined.
Table 6.2: The Punnett Square of a white flower (bb) crossed with a purple flower (Bb).
Lesson Summary
• Gregor Mendel is considered the father of genetics, the science of studying inheritance.
• According to Mendel’s law of segregation, an organism has two factors for each trait, but each gamete only
contains one of these factors.
• A Punnett square is useful for predicting the outcomes of crosses.
Review Questions
1. What is the term for the offspring of a cross, or the first generation?
2. What is the F2 generation?
3. Who is considered the father of genetics?
9. In guinea pigs, black fur (B) is dominant over white fur (b). If a BB guinea pig is crossed with a Bb guinea pig,
what ratio of guinea pigs in the offspring would you predict?
10. In guinea pigs, smooth coat (S) is dominant over rough coat (s). If a SS guinea pig is crossed with a ss guinea
pig, what ratio of guinea pigs in the offspring would you predict?
11. In humans, unattached ear lobes are dominant over attached ear lobes. If two parents have attached earlobes,
what is the predicted ratio in the offspring?
Further Reading / Supplemental Links
• nbs/projects/anthro201/
Points to Consider
• Do you think all traits follow this simple pattern where one factor controls the trait?
• Can you think of other examples where Mendel’s laws do not seem to fit?
Modern Genetics
Lesson Objectives
Compare Mendel’s laws with our modern understanding of chromosomes.
Explain how codominant traits are inherited.
Distinguish between phenotype and genotype.
Explain how polygenic traits are inherited.
Check Your Understanding
• What is a visual representation of a genetic cross?
• What is stated in Mendel’s law of segregation?
codominance A pattern of inheritance where both alleles are equally expressed.
genotype The genetic makeup of a cell or organism, defined by certain alleles for a particular trait.
heterozygous Having two different alleles for a particular trait.
homozygous Having identical alleles for a particular trait.
phenotype The physical appearance that is a result of the genotype.
polygenic inheritance A pattern of inheritance where the trait is controlled by many genes and each dominant
allele has an additive effect.
Mendel and Modern Genetics
Mendel laid the foundation for modern genetics, but there were still a lot of questions he left unanswered. What
exactly are the dominant and recessive factors that determine how all organisms look? And how do these factors
Since Mendel’s time, scientists have discovered the answers to these questions. Genetic material is made out of
DNA. It is the DNA that makes up the hereditary factors that Mendel identified. By applying our modern knowledge
of DNA and chromosomes, we can explain Mendel’s findings and build on them. In this lesson, we will explore the
other connections between Mendel’s work and modern genetics.
Traits, Genes, and Alleles
Recall that our DNA is wound into chromosomes. Each of our chromosomes contains a long chain of DNA that
encodes hundreds, if not thousands, of genes. Each of these genes can have slightly different versions from individual
to individual. These variants of genes are called alleles.
For example, remember that for the height gene in pea plants there are two possible factors. These factors are alleles.
There is a dominant allele for tallness (T) and a recessive allele for shortness (t).
Genotype and Phenotype
Genotype is a way to describe the combination of alleles that an individual has for a certain gene (Table ?? ). For
each gene, an organism has two alleles, one on each chromosome of a homologous pair of chromosomes (think of
it as one allele from mom, one allele from dad). The genotype is represented by letter combinations, such as TT, Tt,
and tt.
When an organism has two of the same alleles for a specific gene, it is homozygous (homo- means "same") for that
gene. An organism can be either homozygous dominant (TT) or homozygous recessive (tt). If an organism has two
different alleles (Tt) for a certain gene, it is known as heterozygous (hetero- means different).
TABLE 6.2: Genotypes
homozygous dominant
homozygous recessive
two of the same allele
one dominant allele and one recessive allele
two dominant alleles
two recessive alleles
TT or tt
Table 6.4: Genotypes
Phenotype is a way to describe the traits you can see. The genotype is like a recipe for a cake, while the phenotype
is like the cake made from the recipe. The genotype expresses the phenotype. For example, the phenotypes of
Mendel’s pea plants were either tall or short, or were purple-flowered or white-flowered.
Can organisms with different genotypes have the same phenotypes? Let’s see.
What is the phenotype of a pea plant that is homozygous dominant (TT) for the tall trait? Tall. What is the phenotype
of a pea plant that is heterozygous (Tt)? It is also tall. The answer is yes, two different genotypes can result in the
same phenotype. Remember, the recessive phenotype will be expressed only when the dominant allele is absent, or
when an individual is homozygous recessive (tt) (Figure 6.4 ).
Exceptions to Mendel’s Laws: Incomplete Dominance and Codominance
In all of Mendel’s experiments, he worked with traits where a single gene controlled the trait. Each also had one
allele that was always dominant to the recessive allele. But this is not always true.
There are exceptions to Mendel’s rules, and these exceptions usually have something to do with the dominant allele.
If you cross a homozygous red flower with a homozygous white flower, according to Mendel’s laws, what color
flower should result from the cross? Either a completely red or completely white flower, depending on which allele
is dominant. But since Mendel’s time, scientists have discovered this is not always the case.
AA Aa aa or T T T t tt will lead
to different phenotypes or different
appearances of the organism
Incomplete Dominance
One allele is NOT always completely dominant over another allele. Sometimes an individual has a phenotype
between the two parents because one allele is not dominant over another. This pattern of inheritance is called
incomplete dominance. For example, snapdragon flowers show incomplete dominance. One of the genes for
flower color in snapdragons has two alleles, one for red flowers and one for white flowers.
A plant that is homozygous for the red allele (RR) will have red flowers, while a plant that is homozygous for the
white allele will have white flowers (WW). But the heterozygote will have pink flowers (RW) (Figure 6.5 ). Neither
the red nor the white allele is dominant, so the phenotype of the offspring is a blend of the two parents.
Pink snapdragons are an example of incomplete dominance.
Another example of incomplete dominance is sickle cell anemia, a disease in which a blood protein called hemoglobin
is produced incorrectly, causing the red blood cells to have a sickle shape. A person that is homozygous recessive (ss)
for the sickle cell trait will have red blood cells that all have the incorrect hemoglobin. A person who is homozygous
dominant (SS) will have normal red blood cells.
What type of blood cells do you think a person who is heterozygous (Ss) for the trait will have? They will have some
misshapen cells and some normal cells (Figure 6.6 ). Both the dominant and recessive alleles are expressed, so the
result is a phenotype that is a combination of the recessive and dominant traits.
Sickle cell anemia causes red blood
cells to become misshapen and curved
right cell unlike normal rounded red
blood cells le f t cell .
Another exception to Mendel’s laws is a called codominance. For example, our blood type shows codominance. Do
you know what your blood type is? Are you A? O? AB? Those letters actually represent alleles. Unlike other traits,
your blood type has 3 alleles, instead of 2!
The ABO blood types (Figure 6.7 ) are named for the protein, or antigen, attached to the outside of the blood cell.
An antigen is a substance that provokes an immune response, your body’s defenses against disease, which will be
discussed further in the Diseases and the Body’s Defenses chapter.
In this case, two alleles are dominant and completely expressed (IA and IB ), while one allele is recessive (i). The
IA allele encodes for red blood cells with the A antigen, while the IB allele encodes for red blood cells with the B
antigen. The recessive allele (i) doesn’t encode for any proteins. Therefore a person with two recessive alleles (ii)
has type O blood. As no dominant (IA and IB ) allele is present, the person cannot have type A or type B blood.
There are two possible genotypes for type A blood, homozygous (IA IA ) and heterozygous (IA i), and two possible
genotypes for type B blood, (IB IB and IB i). If a person is heterozygous for both the IA and IB alleles, they will
express both and have type AB blood with both antigens on each red blood cell.
This pattern of inheritance is significantly different than Mendel’s rules for inheritance because both alleles are
expressed completely and one does not mask the other.
Polygenic Traits and Environmental Influences
Another exception to Mendel’s rules is polygenic inheritance, which occurs when a trait is controlled by more than
one gene. This means that each dominant allele "adds" to the expression of the next dominant allele.
Usually, traits are polygenic when there is wide variation in the trait. For example, humans can be many different
sizes. Height is a polygenic trait. If you are dominant for all of the alleles for height, then you will be very tall.
There is also a wide range of skin color across people. Skin color is also a polygenic trait.
Polygenic inheritance often results in a bell shaped curve when you analyze the population (Figure 6.8 ). That
means that most people fall in the middle of the phenotypic range, such as average height, while very few people are
at the extremes, such as very tall or very short.
An example of codominant inheritance
is ABO blood types.
Polygenic traits tend to result in a distribution that resembles a bell-shaped
curve with few at the extremes and most
in the middle. There may be 4 or 6 or
more alleles involved in the phenotype.
At the left extreme individuals are completely dominant for all alleles and at
the other extreme individuals are completely recessive for all alleles. Individuals in the middle have various combinations of recessive and dominant alleles.
Lesson Summary
• Variants of genes are called alleles.
• Genotype is the combination of alleles that an individual has for a certain gene, while phenotype is the appearance caused by the expression of the genotype.
• Incomplete dominance and codominance do not fit Mendel’s rules because one allele does not entirely mask
the other.
• In polygenic inheritance, many genes control a trait. Each dominant allele adds to the next dominant allele.
Review Questions
1. What is a variant of a gene that occurs at the same place on homologous chromosomes?
2. What is the type of allele that only affects the phenotype in the homozygous condition?
3. What type of allele masks the expression of the recessive allele?
4. What is the term for the specific alleles of an individual for a particular trait?
5. What is the term for the appearance of the organism, as determined by the genotype?
Apply Concepts
6. If two individuals have a certain phenotype, such as tall pea plants, does that mean they must have the same
7. What is the term for the pattern of inheritance where an individual has an intermediate phenotype between the
two parents?
8. What is the inheritance pattern where both alleles are expressed?
Think Critically
9. IQ in humans varies, with most people having an IQ of around 100, and with a few people at the extremes, such
as 50 or 150. What type of inheritance do you think this might describe?
10. A dark purple flower is crossed with a white flower of the same species and the offspring have light purple
flowers. What type of inheritance does this describe? Explain.
Further Reading / Supplemental Links
Points to Consider
• Hypothesize about the genetic differences between males and females.
• Can you name any human genetic disorders?
• If a baby inherits an extra chromosome, what might the result be?
Human Genetics
Lesson Objectives
List the two types of chromosomes in the human genome.
Predict patterns of inheritance for traits located on the sex chromosomes.
Describe how some common human genetic disorders are inherited.
Explain how changes in chromosomes can cause disorders in humans.
Check Your Understanding
• How many alleles does an individual have for each gene/trait?
• How do we predict the probability of traits being passed on to the next generation?
• What do we call complexes of DNA wound around proteins that pass on genetic information to the next
generation of cells?
autosomes The chromosomes other than the sex chromosomes.
pedigree A chart which shows the inheritance of a trait over several generations.
sex-linked inheritance The inheritance of traits that are located on genes on the sex chromosomes.
sex-linked trait A trait that is due to a gene located on a sex chromosome, usually the X-chromosome.
Special Inheritance Patterns
What gene determines if a baby is male or female? How are humans born with genetic disorders, like cystic fibrosis
or Down Syndrome? We can apply Mendel’s rules to describe how many human traits and genetic disorders are
We can now also explain special inheritance patterns that don’t fit Mendel’s rules.
Sex-linked Inheritance
What determines if a baby is a male or female? Recall that you have 23 pairs of chromosomes— and one of those
pairs is the sex chromosomes. Everyone has two sex chromosomes, X or Y. Females have two X chromosomes
(XX), while males have one X chromosome and one Y chromosome (XY).
If a baby inherits an X from the father and an X from the mother, what will be the child’s sex? Female. If the father’s
sperm carries the Y chromosome, the child will be male. Notice that a mother can only pass on an X chromosome,
so the sex of the baby is determined by the father. The father has a 50 percent chance of passing on the Y or X
chromosome, so there is a 50 percent chance that a child will be male, and a 50 percent chance a child will be
One special pattern of inheritance that doesn’t fit Mendel’s rules is sex-linked inheritance, referring to the inheritance of traits that are located on genes on the sex chromosomes. Since males and females do not have the same sex
chromosomes, there will be differences between the sexes in how these sex-linked traits — traits linked to genes
located on the sex chromosomes — are expressed.
One example of a sex-linked trait is red-green colorblindness. People with this type of colorblindness cannot tell the
difference between red and green. They often see these colors as shades of brown (see Figure 6.9 and Table 6.3 ).
Boys are much more likely to be colorblind than girls. This is because colorblindness is a sex-linked recessive trait.
Boys only have one X chromosome, so if that chromosome carries the gene for colorblindness, they will be colorblind. As girls have two X chromosomes, a girl can have one X chromosome with the colorblind gene and one X
chromosome with a normal gene for color vision. Since colorblindness is recessive, the dominant normal gene will
mask the recessive colorblind gene. Females with one colorblindness allele and one normal allele are referred to as
carriers. They carry the allele but do not express it.
How would a female become color-blind? She would have to inherit two genes for colorblindness, which is very
unlikely. Many sex-linked traits are inherited in a recessive manner.
A person with red-green colorblindness
would not be able to see the number.
TABLE 6.3: According to this Punnett square, the son of a woman who carries the colorblindness
trait and a normal male has a 50% chance of being colorblind.
Xc X (carrier female)
Xc Y (colorblind male)
XX (normal female)
XY (normal male)
Table 6.6: According to this Punnett square, the son of a woman who carries the colorblindness trait and a normal
male has a 50% chance of being colorblind.
Human Genetic Disorders
Some human genetic disorders are also X-linked or Y-linked, which means the faulty gene is carried on these sex
chromosomes. Other genetic disorders are carried on one of the other 22 pairs of chromosomes; these chromosomes
are known as autosomes or autosomal (non-sex) chromosomes.
Autosomal Recessive Disorders
Some genetic disorders are caused by recessive or dominant alleles of a single gene on an autosome. An example of
an autosomal recessive genetic disorder is cystic fibrosis. Children with cystic fibrosis have excessively thick mucus
in their lungs, which makes it difficult for them to breathe. The inheritance of this recessive allele is the same as any
other recessive allele, so a Punnett square can be used to predict the probability that two carriers of the disease will
have a child with cystic fibrosis.
What are the possible genotypes of the offspring in Table 6.4 ? What are the possible phenotypes?
TABLE 6.4: According to this Punnett square, two parents that are carriers of cystic fibrosis of
the cystic fibrosis gene have a 25% chance of having a child with cystic fibrosis.
FF (normal)
Ff (carrier)
Ff (carrier)
ff (affected)
According to this Punnett square, two parents that are carriers of cystic fibrosis gene have a 25% chance of having a
child with cystic fibrosis.
Automsomal Dominant Disorders
Huntington’s disease is an example of an autosomal dominant disorder. This means that if the dominant allele is
present, then the person will express the disease.
The disease causes the brain’s cells to break down, leading to muscle spasms and personality changes. Unlike most
other genetic disorders, the symptoms usually do not become apparent until middle age. You can use a simple
Punnett square to predict the inheritance of a dominant autosomal disorder, like Huntington’s disease. If one parent
has Huntington’s disease, what is the chance of passing it on to their children? If you draw the Punnett square, you
will find that there is a 50 percent chance of the disorder being passed on to the children.
Pedigree Analysis
A pedigree is a chart which shows the inheritance of a trait over several generations. A pedigree is commonly
created for families, and it outlines the inheritance patterns of genetic disorders.
Figure 6.10 shows a pedigree displaying recessive inheritance of a disorder through three generations. From studying a pedigree, scientists can determine the following:
• If the trait is sex-linked (on the X or Y chromosome) or autosomal (on a chromosome that does not determine
• If he trait is inherited in a dominant or recessive fashion, and possibly whether individuals with the trait are
heterozygous or homozygous.
In a pedigree squares symbolize males and circles represent females. A horizontal line joining a male and female
indicates that the couple had offspring. Vertical lines indicate offspring which are listed left to right in order of
birth. Shading of the circle or square indicates an individual who has the trait being traced. The inheritance of the
recessive trait is being traced. &#8220 A&#8221 is the dominant allele and &#8220 a&#8221 is the recessive
Pedigree analysis is discussed in (9:13).
Click image to the left for more content.
Chromosomal Disorders
Some children are born with genetic defects that are not carried by a single gene. Instead, an error in a larger part
of the chromosome or even in an entire chromosome causes the disorder. Usually the error happens when the egg or
sperm is forming. Having extra chromosomes or damaged chromosomes can also cause disorders.
Extra Chromosomes
One common example of an extra-chromosome disorder is Down syndrome (Figure 6.11 and Figure 6.12 ). Children with Down syndrome are mentally disabled and also have physical deformities. Down syndrome occurs when
a baby receives an extra chromosome from one of his or her parents. Usually, a child will receive one chromosome
21 from the mother and one chromosome 21 from the father. In an individual with Down syndrome, however, there
are three copies of chromosome 21. Down syndrome is therefore also known as Trisomy 21.
A child with Down syndrome.
Another example of a chromosomal disorder is Klinefelter syndrome, in which a male inherits an extra “X” chromosome. These individuals have underdeveloped sex organs and elongated limbs, and have difficulty learning new
Outside of chromosome 21 and the sex chromosomes, most embryos with extra chromosomes do not usually survive.
Because chromosomes carry many, many genes, a disruption of a chromosome can cause severe problems with the
development of a fetus.
Damaged Chromosomes
Chromosomal disorders also occur when part of a chromosome becomes damaged. For example, if a tiny portion
of chromosome 5 is missing, the individual will have cri du chat (cat’s cry) syndrome. These individuals have
Karyotype of Down Syndrome. Notice
the extra chromosome 21.
misshapen facial features and the infant’s cry resembles a cat’s cry.
Lesson Summary
• Some human traits are controlled by genes on the sex chromosomes.
• Human genetic disorders can be inherited through recessive or dominant alleles, and they can be located on
the sex chromosomes or autosomes (non-sex).
• Changes in chromosome number can lead to disorders like Down syndrome.
Review Questions
1. How many chromosomes do you have in each cell of your body?
Apply Concepts
2. How is Down’s syndrome inherited?
Think Critically
3. A son cannot inherit colorblindness from his father. Why not?
4. One parent is a carrier of the cystic fibrosis gene, while the other parent does not carry the allele. Can their child
have cystic fibrosis?
Further Reading / Supplemental Links
Points to Consider
• Human cloning is illegal in many countries. Do you agree with these restrictions?
• Why would it be helpful to know all the genes that make up human DNA?
• It may be possible in the future to obtain the sequence of all your genes. Would you want to take advantage of
this opportunity? Why or why not?
Genetic Advances
Lesson Objectives
Explain how clones are made.
Explain how vectors are made.
Explain what sequencing a genome tells us.
Describe how gene therapy works.
Check Your Understanding
• What part of the cell contains the genetic material?
• What are the base pairing rules for DNA?
cloning Creating an identical copy of a gene, or an an individual with the same genes.
gene therapy The insertion of genes into a person’s cells to cure a genetic disorder.
Human Genome Project International effort to sequence all the base pairs in human DNA; completed in 2003.
plasmid Small circular piece of DNA; found in prokaryotic cells.
recombinant DNA DNA formed by the combination of DNA from two different sources, such as placing a human
gene into a bacterial plasmid.
somatic cell A body cell; not a gamete.
transformation The process by which bacteria pick up foreign DNA and incorporate it in their genome.
vector An organism that carries pathogens from one person or animal to another.
Biotechnology and DNA Technology
Since Mendel’s time, there have been many changes in the understanding of genetics. As scientists learn more about
how DNA works, they can develop technologies that allow us to reveal the genetic secrets encoded in our DNA and
even alter an organism’s DNA.
Genetic engineering (also known as "biotechnology" or "DNA technology") helps us better understand and predict
the inheritance of genetic diseases, treat these diseases, produce new medicines, and even produce new food products.
Biotechnology: An Introduction can be viewed at;feature=rela
ted (1:05).
Click image to the left for more content.
Recombinant DNA
Recombinant DNA is the combination of DNA from two different sources. For example, it is possible to place
a human gene into bacterial DNA. Recombinant DNA technology is useful in gene cloning and in identifying the
function of a gene, as well as producing useful proteins, such as insulin. To treat diabetes, many people need insulin.
Previously, insulin had been taken from animals. Through recombinant DNA technology, bacteria were created that
carry the human gene which codes for the production of insulin. These bacteria become tiny factories that produce
this protein. Recombinant DNA technology helps create insulin so it can be used by humans.
A clone is an exact copy. Below are the steps used to copy, or clone, a gene:
a. A gene or piece of DNA is put in a vector, or carrier molecule, producing a recombinant DNA molecule.
b. The vector is placed into a host cell, such as a bacterium.
c. The gene is copied (or cloned) inside of the bacterium. As the bacterial DNA is copied, so is the vector DNA.
As the bacteria divide, the recombinant DNA molecules are divided between the new cells. Over a 12 to 24
hour period, millions of copies of the cloned DNA are made.
d. The cloned DNA can produce a protein (like insulin) that be used in medicine or in research.
Bacteria have small rings of DNA in the cytoplasm, called plasmids (Figure 6.13 ). When putting foreign DNA into
a bacterium, the plasmids are often used as a vector. Viruses can also be used as vectors. Try inserting DNA into a
fly embryo here:
Cloning is the process of creating an exact replica of an organism. The clone’s DNA is exactly the same as the
parent’s DNA. Bacteria and plants have long been able to clone themselves through asexual reproduction. In animals,
however, cloning does not happen naturally. In 1997, that all changed when a sheep named Dolly was the first
mammal ever to be successfully cloned. Other animals can now also be cloned in a laboratory.
The process of producing an animal like Dolly starts with a single cell from the animal that is going to be cloned.
Below are the steps involved in the process of cloning:
a. In the case of Dolly, cells from the mammary glands were taken from the adult that was to be cloned. But
other somatic cells can be used. Somatic cells come from the body and are not gametes like sperm or egg.
b. The nucleus is removed from this cell.
This image shows a drawing of a plasmid. The plasmid has two large segments and one small segment depicted.
The two large segments green and blue
indicate antibiotic resistances usually
used in a screening procedure and the
small segment red indicates an origin
of replication. The origin of replication
is where DNA replication starts copying
the plasmid. The antibiotic resistance
segments ensure only bacteria with the
plasmid will grow.
c. The nucleus is placed in a donor egg that has had its nucleus removed.
d. The new cell is stimulated with an electric shock and embryo development begins, as if it were a normal
e. The resulting embryo is implanted into a mother sheep, where it continue its development. This process is
shown in Figure 6.14 .
animal&#8217 s
f rom which the nucleus has been removed
from a donor.
Is Cloning Easy?
Cloning is not always successful. Most of the time, this cloning process does not result in a healthy adult animal.
The process has to be repeated many times until it works. In fact, 277 tries were needed to produce Dolly. This high
failure rate is one reason that human cloning is banned in the United States. In order to produce a cloned human,
many attempts would result in the surrogate mothers experiencing miscarriages, stillbirths, or deformities in the
infant. There are also many additional ethical considerations related to human cloning. Can you think of reasons
why people are for or against cloning?
Human Genome Project
A person’s genome is all of his or her genetic information; in other words, the human genome is all the information
that makes us human.
The Human Genome Project (Figure 6.15 ) was an international effort to sequence all 3 billion bases that make up
our DNA and to identify within this code more than 20,000 human genes. Scientists also completed a chromosome
map, identifying where the genes are located on each of the chromosomes. The Human Genome Project was completed in 2003. Though the Human Genome Project is finished, analysis of the data will continue for many years.
To complete the Human Genome
Project all 23 pairs of chromosomes
in the human body were sequenced.
Each chromosome contains thousands
of genes. This is a karyotype a visual
representation of an individual&#8217 s
chromosomes lined up by size.
Exciting applications of the Human Genome Project include the following:
• The genetic basis for many diseases can be more easily determined, and now there are tests for over 1,000
genetic disorders.
• The technologies developed during this effort, and since the completion of this project, will reduce the cost of
sequencing a person’s genome. This may eventually allow many people to sequence their individual genome.
• Analysis of your own genome could determine if you are at risk for specific diseases.
• Knowing you might be genetically prone to a certain disease would allow you to make preventive lifestyle
changes or have medical screenings.
The Human Genome Project. Exploring Our Molecular Selves can be viewed at
?v=VJycRYBNtwY#38;feature=related (3:41).
Click image to the left for more content.
Gene Therapy
Gene therapy is the insertion of genes into a person’s cells to cure a genetic disorder. Though gene therapy is still
in experimental stages, the common use of this therapy may occur during your lifetime.
There are two main types of gene therapy:
a. One done inside the body (in vivo)
b. One done outside the body (ex vivo)
In Vivo Gene Therapy
During in vivo gene therapy, done inside the body, the vector with the gene of interest is introduced directly into the
patient and taken up by the patient’s cells. For example, cystic fibrosis gene therapy is targeted at the respiratory
system, so a solution with the vector can be sprayed into the patient’s nose. Recently, in vivo gene therapy was also
used to partially restore the vision of three young adults with a rare type of eye disease.
Ex Vivo Gene Therapy
In ex vivo gene therapy, done outside the body, cells are removed from the patient and the proper gene is inserted
using a virus as a vector. The modified cells are placed back into the patient.
One of the first uses of this type of gene therapy was in the treatment of a young girl with a rare genetic disease,
adenosine deaminase deficiency, or ADA deficiency. People with this disorder are missing the ADA enzyme, which
breaks down a toxin called deoxyadenosine. If the toxin is not broken down, it accumulates and destroys immune
cells. As a result, individuals with ADA deficiency do not have a healthy immune system to fight off infections. In
the gene therapy treatment for this disorder, bone marrow stem cells were taken from the girl’s body and the missing
gene was inserted in these cells outside the body. Then the modified cells were put back into her bloodstream. This
treatment successfully restored the function of her immune system, but only with repeated treatments.
Biotechnology in Agriculture
Biotechnology has also led scientists to develop useful applications in agriculture and food science. These include
the development of transgenic crops - the placement of genes into plants to give the crop a beneficial trait. Benefits
Improved yield from crops.
Reduced vulnerability of crops to environmental stresses.
Increased nutritional qualities of food crops.
Improved taste, texture or appearance of food.
Reduced dependence on fertilizers, pesticides and other chemicals.
Crops are obviously dependent on environmental conditions. Drought can destroy crop yields, as can too much rain
or floods. But what if crops could be developed to withstand these harsh conditions?
Biotechnology will allow the development of crops containing genes that will enable them to withstand harsh conditions. For example, drought and salty soil are two significant factors affecting crop productivity. But there are
crops that can withstand these harsh conditions. Why? Probably because of that plant’s genetics. So scientists are
studying plants that can cope with these extreme conditions, trying to identify and isolate the genes that control these
beneficial traits. The genes could then be transferred into more desirable crops, with the hope of producing the same
phenotypes in those crops.
Thale cress (Figure 6.16 ), a species of Arabidopsis (Arabidopsis thaliana), is a tiny weed that is often used for plant
research because it is very easy to grow and its DNA has been mapped.
Scientists have identified a gene from this plant, At-DBF2, that gives the plant resistance to some environmental
stresses. When this gene is inserted into tomato and tobacco cells, the cells were able to withstand environmental
stresses like salt, drought, cold and heat far better than ordinary cells. If these results prove successful in larger trials,
then At-DBF2 genes could help in engineering crops that can better withstand harsh environments.
Thale cress Arabidopsis thaliana.
Lesson Summary
• Using recombinant DNA technology, a foreign gene can be inserted into an organism’s DNA.
• Cloning of mammals is still being perfected, but several cloned animals have been created by implanting the
nucleus of a somatic cell into a cell in which the nucleus has been removed.
• The Human Genome Project produced a genetic map of all the human chromosomes and determined the
sequence of every base pair in our DNA.
• Gene therapy involves treating an illness caused by a defective gene through the use of a vector to integrate a
normal copy of the gene into the patient.
Review Questions
1. What is the term for all the genetic information of the human species?
2. What are the rings of accessory DNA in bacteria that are often used as a vector in genetic engineering?
3. What is the term for producing identical copies of an organism?
4. What is the vehicle used to introduce foreign DNA into an organism?
Apply Concepts
5. What supplies the cytoplasm of the clone’s cells during the cloning of an organism?
6. What is one application of recombinant DNA technology?
7. Can gene therapy cure a disease caused by a virus?
Critical Thinking
8. What are the benefits of having access to your entire genetic code? What are the possible problems?
9. Should human cloning be legal in the US? Why or why not?
Further Reading / Supplemental Links
Points to Consider
Next we begin to discuss evolution, the change in species over time.
• Fossils provide evidence of evolution, but what is a fossil?
• If two animals are similar in structure, would you guess they are closely related? Why or why not?
MS Evolution
Does this frog look a little scary? It looks that way on purpose. This frog is a poisonous dart frog. They live
in Central and South America. Why do you think the frog is so brightly colored? Why do you think the frog is
poisonous? Why does the frog only live in warmer climates? There are also many different types of poisonous dart
frogs. Some are red, some blue, some yellow. So why is there such a great diversity of poisonous dart frogs?
Scientists who study evolution are concerned with these types of questions, but they ask them about all of the species
on the planet. Why are there millions of different types of species? Why are some small, some large, some furry,
and some covered in feathers? These questions will be explored as we learn about the theory of evolution.
Evolution by Natural Selection
Lesson Objectives
• Explain how evolution is the change of an inherited trait in a population over many generations.
• Explain how evolution is caused by the process of natural selection.
• Describe how both Darwin and Wallace developed the theory of evolution by natural selection at the same
Check Your Understanding
• What does the word "inherit" mean?
• Why do offspring have some of the same traits as their parents?
acquired trait A feature that an organism gets during its lifetime in response to the environment (not from genes);
not passed on to future generations through gene
adaptation A beneficial trait that helps an organism survive in its environment.
artificial selection Occurs when humans select which plants or animals to breed to pass specific traits on to the
next generation.
evolution The process in which something passes to a different stage, such as a living organism turning into a
more advanced or mature organism; the change of the inherited traits of a group of organisms over many
Galápagos Islands A group of islands in the Pacific Ocean off South America; known for unusual animal life.
Many scientists, including Charles Darwin, made many discoveries that led to and support the theory of
evolution by natural selection, while studying the plants and animals on these islands.
inherited traits Features that are passed from one generation to the next.
natural selection Causes beneficial heritable traits to become more common in a population, and unfavorable
heritable traits become less common.
trait A feature or characteristic of an organism; for example, your height, hair color, and eye shape are physical
Darwin’s Observations and The Theory of Evolution
Do you ever wonder why some birds are big like ostriches and some birds are small like robins? Or why a lion has
a mane while a leopard has spots? In the 19th century, an English natural scientist named Charles Darwin (Figure
7.1 ) was also fascinated by the diversity of living things on earth.
He set out to answer the following questions:
• Why are organisms different?
• Why are organisms similar?
• Why are there so many different types of organisms?
To answer his questions, he developed what we now call "the theory of evolution by natural selection." This theory
is one of the most important theories in the field of life science. In everyday English, "evolution" simply means
"change". In biology, evolution is the process that explains why species change over time. Darwin spent over 20
years traveling around the world and making observations before he fully developed his theory.
Charles Darwin was one of the most influential scientists who has ever lived.
Darwin introduced the world to the theory of evolution by natural selection
which laid the foundation for how we understand the living world today.
Voyage of the
In 1859, Charles Darwin published his book, On the Origin of Species by Means of Natural Selection. His book
describes the observations and evidence that he collected over 20 years of research, beginning with a five-year
voyage around the world on a British research ship, the HMS Beagle. During the voyage (Figure 7.2 ), Darwin
made observations about plants and animals around the world. He also collected specimens to study when he
returned to England.
Each time the Beagle stopped at a port, Darwin went on land to explore and look at the local plants, animals, and
fossils. One of the most important things Darwin did was keep a diary. He took detailed notes and made drawings.
Charles Darwin&#8217 s famous five year voyage was aboard the HMS Beagle from 1831-1836.
The Galápagos Islands
While the crew of the HMS Beagle mapped the coastline of South America, they traveled to a group of islands called
the Galápagos. The Galápagos are a group of 16 volcanic islands near the equator, about 600 miles from the west
coast of South America. Darwin spent months on foot exploring the islands. The specimens he collected from the
Galápagos and sent back to England greatly influenced his ideas of evolution (Figure 7.3 ).
On the Galapagos, Darwin observed that the same kind of animal differed from one island to another. For example,
the iguanas (large lizards) differed between islands (Figure 7.4 ). The members of one iguana species spent most of
their time in the ocean, swimming and diving underwater for seaweed, while those of another iguana species lived
on land and ate cactus. Darwin wondered why there were two species of iguana on the same set of islands that were
so different from one another. What do you think?
Giant Tortoises
Darwin also observed giant tortoises on the Galápagos (Figure 7.5 ). These tortoises were so large that two people
could ride on them. Darwin noticed that different tortoise species lived on islands with different environments. He
realized that the tortoises had traits that allowed them to live in their particular environments. For example, tortoises
that ate plants near the ground had rounded shells and shorter necks. Tortoises on islands with tall shrubs had longer
necks and shells that bent upwards, allowing them to stretch their necks (Figure 7.6 ). Darwin began to hypothesize
that organisms developed traits over time because of differences in their environments.
The Gal&#225 pagos Islands are a
group of 16 volcanic islands 600 miles
off the west coast of South America.
The islands are famous for their many
species found nowhere else.
Gal&#225 pagos
among the signature animals of the
Gal&#225 pagos Islands. Here both a
land iguana and a marine iguana are
&#8220 Gal&#225 pagos&#8221
means &#8220 giant tortoise.&#8221
Gal&#225 pagos
amazed by the size and variety of
shapes of these animals. The giant
tortoise is a unique animal found only in
the Gal&#225 pagos Islands. There are
only about 200 tortoises remaining on
these islands.
This tortoise is able to reach leaves high
in shrubs with its long neck and curved
Darwin’s Finches
The most studied animals on the Galápagos are finches, a type of bird (Figure 7.7 ). When Darwin first observed
finches on the islands, he did not even realize they were all finches. But when he studied them further, he realized
they were related to each other. Each island had its own distinct species of finch. The birds on different islands had
many similarities, but their beaks differed in size and shape.
Four of Darwin&#8217 s finch species
from the Gal&#225 pagos Islands. The
birds came from the same finch ancestor. They evolved as they adapted to
different food resources on different islands. The first bird uses its large beak
to crack open and eat large seeds. Bird
#3 is able to pull small seeds out of
small spaces.
In his diary, Darwin pointed out how each animal is well-suited for its particular environment. The shapes of the
finch beaks on each island were well-matched with the seeds available on that island, but not the seeds on other
islands. For example, a larger and stronger beak was needed to break open large seeds on one island and a small
beak was needed to eat the small seeds on a different island.
For a video of naturalist Sir David Attenborough on Charles Darwin and evolution, see
atch?v=uz7U4k522Pg (4:27).
Click image to the left for more content.
An overview of evolution can be seen at
GcjgWov7mTM (17:39).
Click image to the left for more content.
Darwin’s ideas can be summarized in the Natural Selection and the Owl Butterfly video:
ser/khanacademy#p/c/7A9646BC5110CF64/3/dR_BFmDMRaI (13:29).
Click image to the left for more content.
Influences on Darwin
When Darwin returned to England five years later, he did not rush to announce his discoveries. Unlike other naturalists before him, Darwin did not want to present any ideas unless he had strong evidence supporting them. Instead,
once Darwin returned to England, he spent over twenty years examining specimens, talking with other scientists and
collecting more information before he presented his theories.
Some of Darwin’s ideas conflicted with widely held beliefs, including those from religious leaders, such as:
• All organisms never change and never go extinct.
• The world is only about 6,000 years old.
These beliefs delayed Darwin in presenting his findings. How did Darwin come up with his theories? Charles
Darwin was influenced by the ideas of several people. Before the voyage of the Beagle, Jean-Baptiste Lamarck
proposed the idea that evolution occurs. However, Darwin differed with Lamarck on several key points. Lamarck
proposed that traits acquired during one’s lifetime could be passed to the next generation. Darwin did not agree with
this. The findings of Charles Lyell, a well-known geologist, also influenced Darwin. Lyell taught Darwin about
geology, paleontology and the changing Earth. Lyell’s findings suggested the Earth must be much older than 6,000
After the Voyage of the Beagle, another naturalist, Alfred Wallace (Figure 7.8 ), developed a similar theory of
evolution by natural selection. Wallace toured South America and made similar observations to Darwin’s. Darwin
and Wallace presented their theories and evidence in public together. Due to the large number of observations and
conclusions he made, Darwin is mostly credited and associated with this theory.
Alfred Wallace developed a similar theory of evolution by natural selection.
Natural Selection and Adaptation
The theory of evolution by natural selection means that the inherited traits of a population change over time through
a process called natural selection. Inherited traits are features that are passed from one generation to the next. For
example, your eye color is an inherited trait (you inherited it from your parents). Inherited traits are different from
acquired traits, or traits that organisms develop over a lifetime, such as strong muscles from working out (Figure
7.9 ).
Human earlobes may be free or attached. You inherited the particular
shape of your earlobes from your parents. Inherited traits are influenced by
genes which are passed on to offspring
and future generations. Your summer
tan is not passed on to your offspring.
Natural selection only operates on traits
like earlobe shape that have a genetic
basis not on traits like a summer tan
that are acquired.
Natural selection explains how organisms in a population develop traits that allow them to survive and reproduce.
These traits will most likely be passed on to their offspring. Evolution occurs by natural selection. Take the giant
tortoises on the Galápagos as an example. If a short-necked tortoise lives on an island with fruit located at a high
level, will the short-necked tortoise survive? No, it will not, because it will not be able to reach the food it needs
to survive. If all of the short necked tortoises die, and the long-necked tortoises survive, then over time only the
long-necked trait will be passed down to offspring. All of the tortoises with long-necks will be "naturally selected"
to survive.
Every plant and animal depends on its traits to survive. Survival may include getting food, building homes, and
attracting mates. Traits that allow a plant, animal, or bacteria to survive and reproduce in its environments are called
Natural selection occurs when:
a. There is some variation in the inherited traits of organisms within a species.
b. Some of these traits will give individuals an advantage over others in surviving and reproducing.
c. These individuals will be likely to have more offspring.
Imagine how in winter, dark fur makes a rabbit easy for foxes to spot and catch in the snow. Natural selection
suggests that white fur is a beneficial trait that improves the chance that a rabbit will survive, reproduce and pass the
trait of white fur on to its offspring (Figure 7.10 ). Over time, dark fur rabbits will become uncommon. Rabbits will
adapt to have white fur.
In winter the fur of Arctic hares turns
white. The camouflage may make it
more difficult for fox and other predators
to locate hares against the white snow.
Why so many species?
Scientists estimate that there are between 5 million and 100 million species on the planet. But why are there so
many? As environments change over time, organisms must constantly adapt to those environments. Diversity of
species increases the chance that at least some organisms adapt and survive any major changes in the environment.
For example, if a natural disaster kills all of the large organisms on the planet, then the small organisms will continue
to survive.
Lesson Summary
• Evolution is a change in species over multiple generations.
• Natural selection is how evolution occurs when organisms develop traits that allow them to survive, reproduce,
and pass on their traits to their offspring.
• Adaptations are the result of natural selection.
• Charles Darwin is credited with developing the theory of evolution by natural selection.
The Simpsons - Homer Evolution can be viewed at (1:30).
Click image to the left for more content.
Review Questions
1. Define biological evolution.
2. What was the name of the ship that Darwin traveled on?
3. What is the name of the islands where Darwin studied evolution?
4. Who proposed a theory of evolution by natural selection that was similar to Darwin’s theory?
Apply Concepts
5. How is evolution the result of natural selection?
6. What is an example of an adaptation?
7. What is the difference between an inherited trait and an acquired trait?
8. A giraffe’s long neck allows the giraffe to eat leaves from high in the tree. This is an example of an _____________.
Critical Thinking
9. If a species of finch lives on an island with small seeds that fall easily into cracks between rocks, what kind of
beak traits will be selected for over time?
Further Reading / Supplemental Links
Stein, Sara, The Evolution Book, Workman, N.Y., 1986.
Yeh, Jennifer, Modern Synthesis, (From Animal Sciences).
Darwin, Charles, Origin of the Species, Broadview Press (Sixth Edition), 1859.
Ridley, Matt, The Red Queen: Sex and the Evolution of Human Nature, Perennial Books, 2003.
Ridley, Matt, Genome, Harper Collins, 2000.
Sagan, Carl, Cosmos, Edicions Universitat Barcelona, 2006.
Carroll, Sean B., The Making of the Fittest: DNA and the Ultimate Forensic Record of Evolution, Norton,
Dawkins, Richard, The Blind Watchmaker, W.W. Norton #38; Company, 1996.
Dawkins, Richard, The Selfish Gene, Oxford University Press, 1989.
Diamond, Jared, The Third Chimpanzee: The Evolution and Future of the Human Animal, HarperCollins,
Mayr, Ernst, What Evolution Is, Basic Books, 2001.
Zimmer, Carl, Smithsonian Intimate Guide to Human Origins, Smithsonian Press, 2008.
Charles Darwin and the Tree of Life
Part 1:
Click image to the left for more content.
Part 4:;feature=related
Click image to the left for more content.
Part 5:;feature=fvwrel
Click image to the left for more content.
Points to Consider
• What kind of evidence supports the theory of evolution by natural selection?
• How does genetics provide a basis for Darwin’s original observations?
Evidence of Evolution
Lesson Objectives
Explain how the scientific theory of biological evolution is based on physical evidence and experiments.
Discuss the significance of the fossil record as evidence for evolution.
Compare vestigial structures and embryos as a biological basis for evolution.
Explain the genetic basis for evolution.
Check Your Understanding
• Where did Charles Darwin collect evidence of evolution and what kinds of evidence did he find?
• What is natural selection?
• What kinds of traits change through evolution?
embryology The study of how organisms develop.
fossil The preserved remains or traces of animals, plants, and other organisms from the distant past; examples
include bones, teeth, impressions, and leaves.
fossil record Fossils and the order in which fossils appear; provides important records of how species have
evolved, divided and gone extinct.
genome All of the genes in an organism.
paleontologists Scientists who study fossils to learn about life in the past.
radiometric dating A method to determine the age of rocks and fossils in each layer of rock; measures the decay
rate of radioactive materials in each rock layer.
vestigial structure Body part that, through evolution, has lost its use, such as a whale’s pelvic bones.
The Fossil Record
Fossils are preserved parts of animals, plants, and other organisms from the distant past. Examples of fossils include
bones, teeth, and impressions. By studying fossils, evidence for evolution is revealed.
Paleontologists are scientists who study fossils to learn about life in the past. Paleontologists compare the features
of species from different periods in history. With this information, they try to understand how species have evolved
over millions of years (Figure 7.11 ).
Until recently, fossils were the main source of evidence for evolution (Figure 7.12 and Figure ?? ).
• The location of each fossil in layers of rock provides clues to the age of the species and how species evolved
in the past.
• In the past, organisms were spread out differently across the planet. Fossils also allow us to understand how
earthquakes, volcanoes, shifting seas, and other movements of the continents affect where organisms once
lived and how they adapted to their changing environments.
Rock Layers and the Age of Fossils
There are many layers of rock in the Earth’s surface. These layers provide evidence for when organisms lived on
Earth, how species evolved, and how some species have gone extinct. The fossils and the order in which fossils
appear is called the fossil record.
Geologists use a method called radiometric dating to determine the age of rocks and fossils in each layer of rock.
This technique measures the how fast the radioactive materials in each rock layer are broken down (Figure 7.13 ).
Radiometric dating has been used to determine that the oldest known rocks on Earth are between 4 and 5 billion
years old. The oldest fossils are between 3 and 4 billion years old. Remember that during Darwin’s time, people
believed the earth was just about 6,000 years old. The fossil record proves that Earth is much older than people once
Vestigial Structures
Millions of species of organism are alive today. Even though two different species may not look similar, they may
have similar internal structures that suggest they have a "common ancestor," or organism that they both evolved from
a long time ago.
Some of the most interesting kinds of evidence for evolution are body parts that have lost their use through evolution
(Figure 7.14 ). For example, most birds need their wings to fly. But the wings of an ostrich have lost their original
use. Structures that have lost their use through evolution are called vestigial structures. They provide evidence for
evolution because they suggest that an organism changed from using the structure to not using the structure, or using
it for a different purpose. Penguins do not use their wings to fly in the air; however they do use them to "fly" in the
water. The theory of evolution suggests that penguins evolved to use their wings for a different purpose. A whale’s
pelvic bones, which were once attached to legs, are also vestigial structures (Figure 7.15 ).
Similar Embryos
Some of the oldest evidence of evolution comes from embryology, the study of how organisms develop. An embryo
is an animal or plant in its earliest stages of development, before it is born or hatched. Centuries ago, people
recognized that the embryos of many different species have similar appearances (Figure ?? ). The embryos of some
species are even difficult to tell apart. Many of these animals do not differ much in appearance until they develop
Evolution of the horse. Fossil evidence depicted by the skeletal fragments demonstrates evolutionary milestones
in this process. Notice the 57 million year evolution of the horse leg bones and teeth. Especially obvious is the
transformation of the leg bones from having four distinct digits to that of todayś horse.
A fossil is the remains of a plant or animal that existed some time in the distant past. Fossils such as this one were
found in rocks or soil that formed long
This device called a spectrophotometer
can be used to measure the level of radioactive decay of certain elements in
rocks and fossils to determine their age.
Mole rats live under ground where they
do not need eyes to find their way
around. This mole&#8217 s eyes are
covered by skin. Body parts that do not
serve their original function are vestigial
The bones in your arms and hands have
the same bone pattern as those in the
wings legs and feet of the animals pictured above. The bone structures clockwise from top left are salamander turtle
crocodile bird human mole whale bat.
How have the bones adapted for different uses in each animal
Many traits of one type of animal appear in the embryo of another type of animal. For example, fish embryos and
human embryos both have gill slits. In fish they develop into gills, but in humans they disappear before birth (Figure
?? ).
The similarities between embryos suggests that these animals are related and have common ancestors. For example,
humans did NOT evolve from chimpanzees. But the similarities between the embryos of both species suggest that
we have an ancestor in common with chimpanzees. As our common ancestor evolved, humans and chimpanzees
went down different evolutionary paths and developed different traits.
Vertebrate Embryos. Embryos of different vertebrates look much more similar
than the adult organisms do.
Similar Molecules
Arguably, some of the best evidence of evolution comes from examining the molecules and DNA found in all
organisms (Figure 7.17 ).
In the 1940’s, scientists studying molecules and DNA confirmed conclusions about evolution drawn from other
forms of evidence. Molecular clocks are used to determine how closely two species are related by calculating the
number of differences between the species’ DNA sequences or amino acid sequences. These clocks are sometimes
called gene clocks or evolutionary clocks. The fewer the differences, the less time since the species split from each
other and began to evolve into different species. For example, a chicken and a gorilla will have more differences
between their DNA and amino acid sequences then a gorilla and an orangutan. This provides additional evidence
that the gorilla and orangutan are more closely related than the gorilla and the chicken.
Similar Genetics
The study of genetics has revealed the record of evolution left in the genomes of all organisms (Figure 7.18 ). It also
provides new information about the relationships among species and how evolution occurs.
all o f the genes in an organism
of all mammals are almost identical.
Genetic evidence for evolution includes:
a. The same biochemical building blocks – such as amino acids and nucleotides - are responsible for life in all
organisms, from bacteria to plants and animals.
b. DNA and RNA determine the development of all organisms.
c. The similarities and differences between the genomes reveal patterns of evolution.
This is a map of the genes on just
one of the 46 human chromosomes.
Similarities and differences between the
genomes the genetic makeup of different organisms reveal the relationships between the species. The human
and chimpanzee genomes are almost
identical- there is only a difference of
1.2% between the two genomes.
Lesson Summary
• Fossils provide evidence of how different organisms exist as environmental conditions change over time.
• Radiometric dating has been used to determine that the oldest known rocks on Earth are between 4-5 billion
years old. The oldest fossils are between 3-4 billion years old.
• Vestigial structures indicate that two species have a common ancestor.
• The similarities between embryos suggests that animals are related and have common ancestors.
• The same biochemical building blocks – such as amino acids and nucleotides - are responsible for life in all
organisms, from bacteria to plants and animals.
• The similarities and differences between the genomes reveal patterns of evolution.
Review Questions
1. What are the different kinds of evidence of evolution? How do geologists determine the age of rocks and fossils?
2. What is a vestigial structure?
3. What is an embryo?
4. What is a molecular clock?
Apply Concepts
5. What is an example of a vestigial structure?
6. How do the embryos of different species support the theory of evolution?
7. How do similarities between molecules support the theory of evolution?
Critical Thinking
8. If a type of bird in California is 80% genetically similar to a bird in Nevada, do you think they evolved from a
common ancestor? Why or why not?
9. How does one recent scientific advancement support a part of Darwin’s Theory of Evolution by Natural Selection?
Further Reading / Supplemental Links
Stein, Sara, The Evolution Book, Workman, N.Y., 1986.
Yeh, Jennifer, Modern Synthesis, (From Animal Sciences).
Darwin, Charles, Origin of the Species, Broadview Press (Sixth Edition), 1859 .
Ridley, Matt, The Red Queen: Sex and the Evolution of Human Nature. Perennial Books, 2003.
Ridley, Matt, Genome, Harper Collins, 2000.
Sagan, Carl, Cosmos, Edicions Universitat Barcelona, 2006.
Carroll, Sean B., The Making of the Fittest: DNA and the Ultimate Forensic Record of Evolution, Norton,
Dawkins, Richard, The Blind Watchmaker, W.W. Norton #38; Company, 1996.
Dawkins, Richard, The Selfish Gene, Oxford University Press, 1989.
Diamond, Jared, The Third Chimpanzee: The Evolution and Future of the Human Animal, HarperCollins,
Mayr, Ernst, What Evolution Is, Basic Books, 2001.
Zimmer, Carl, Smithsonian Intimate Guide to Human Origins, Smithsonian Press, 2008.
Points to Consider
• How do you think new species evolve?
• How long do you think it takes for a new species to evolve?
Lesson Objectives
• Compare and contrast microevolution and macroevolution.
• Define speciation as the formation of new species.
• Explain the ways that speciation can occur.
Check Your Understanding
• Why can’t an individual person evolve? Why can only groups evolve over many generations?
• What causes a species or a population to evolve?
allopatric speciation Speciation that occurs when groups from the same species are geographically isolated physically for long periods.
behavioral isolation The separation of a population from the rest of its species due to some behavioral barrier,
such as having different mating seasons.
evolutionary tree Diagram used to represent the relationships between different species and their common ancestors.
geographic isolation The separation of a population from the rest of its species due to some physical barrier, such
as a mountain range, an ocean, or great distance.
macroevolution Big evolutionary changes that result in new species.
microevolution Small changes in inherited traits; does not lead to the creation of a new species.
population A group of organisms belonging to the same species, that live in the same area, and interact with one
reproductive isolation Allopatric and sympatric speciation; isolation due to geography or behavior, resulting in
the inability to reproduce.
speciation The creation of a new species; either by natural or artificial selection.
sympatric speciation Speciation that occurs when groups from the same species stop interbreeding, because of
something other than physical separation, such as behavior.
Microevolution and Macroevolution
A species is a group of organisms that have similar characteristics and can mate with one another to produce offspring. The offspring of two members of the same species must also be able to reproduce. So, how are new species
created? Through the process of evolution.
A population is a group of organisms of the same species that live in the same area (Figure 7.19 ). But does
evolution happen as small changes build up in a population of organisms over time and gradually lead to a new
species? Or is it possible that drastic environmental changes rapidly give rise to new species? Or can both small and
large changes cause evolution to occur?
This school of fish are considered a
species because they are able to mate
with one another. But they are considered a population because they live in
the same part of the ocean.
You already know that evolution is the change in species over time. Most evolutionary changes are small and do
not lead to the creation of a new species. When organisms change in small ways over time, the process is called
An example of microevolution is the evolution of mosquitoes that cannot be killed by pesticides, called pesticideresistant mosquitoes. Imagine that you have a pesticide that kills most of the mosquitoes in your state. As a result,
the only remaining mosquitoes are the pesticide-resistant mosquitoes. When these mosquitoes reproduce the next
year, they produce more mosquitoes with the pesticide-resistant trait.
This is an example of microevolution because the number of mosquitoes with this trait changed. However, this
evolutionary change did not create a new species of mosquito because the pesticide-resistant mosquitoes can still
reproduce with other non-pesticide-resistant mosquitoes.
Macroevolution refers to much bigger evolutionary changes that result in new species. Macroevolution may happen:
a. When microevolution occurs for a long period of time and leads to the creation of a new species.
b. As a result of a major environmental change, such as a volcanic eruption, earthquake, or asteroid hitting Earth,
which changes the environment so much that natural selection leads to large changes in the traits of a species.
After thousands of years of isolation from each other, Darwin’s finch populations have experienced both microevolution and macroevolution. These finch populations cannot breed with other finch populations when they are brought
together. Since they do not breed together, they are classified as separate species.
Evolution Acts on the Phenotype
Natural selection acts on the phenotype - the traits or characteristics - of an individual, not on the underlying genotype. For many traits, the homozygous genotype, AA for example, has the same phenotype as the heterozygous Aa
genotype. If both an AA and Aa individual have the same phenotype, the environment cannot distinguish between
them. So natural selection cannot choose a homozygous individual over a heterozygous individual. The recessive a allele will be maintained in the population through heterozygous Aa individuals. Thus, the mating of two
heterozygous individuals can produce homozygous recessive (aa) individuals.
Since natural selection acts on the phenotype, if an allele causes death in a homozygous individual, aa for example,
it will not cause death in a heterozygous Aa individual. These heterozygous Aa individuals will then act as carriers
of the a allele, meaning that the "a" allele could be passed down to offspring. This allele is said to be kept in the
population’s gene pool. The gene pool is the complete set of alleles within a population.
Tay-Sachs disease is an autosomal recessive genetic disorder. It is caused by the homozygous recessive genotype,
Affected individuals usually die from complications of the disease in early childhood. If the parents are each heterozygous (Rr) for the Tay-Sachs, they will not die, but they will be carriers. If you create a Punnett Square, what
are the chances of a child inheriting the disorder? This deadly allele is kept in the gene pool even though it does not
help humans adapt to their environment. This happens because evolution acts on the phenotype, not the genotype
(Figure 7.20 ).
Hardy-Weinberg Equilibrium
The Hardy-Weinberg model states that a population will remain at genetic equilibrium, with no evolution, as long
as five conditions are met:
No change in the DNA sequence
No migration (moving into or out of a population)
A very large population size
Random mating
No natural selection
These five conditions rarely occur in nature. When one of the conditions exists, then evolution can occur. The HardyWeinberg model is a mathematical formula used to predict allele frequencies in a population at genetic equilibrium.
A video explanation of the Hardy-Weinberg model can be viewed at
/7A9646BC5110CF64/14/4Kbruik_LOo (14:57).
Tay-Sachs disease is inherited in the autosomal recessive pattern. Each parent is an unaffected carrier of the
lethal allele.
Click image to the left for more content.
The Origin of Species
The creation of a new species is called speciation. Most new species develop naturally, but humans have also
artificially created new breeds and species for thousands of years. Natural selection causes beneficial heritable traits
to become more common in a population, and unfavorable heritable traits to become less common. For example, a
giraffe’s neck is beneficial because it allows the giraffe to reach leaves high in trees. Natural selection caused this
beneficial trait to become more common than short necks.
As new changes in the DNA sequence are constantly being generated in a population’s gene pool, some of these
changes will be beneficial and result in traits that allow adaptation and survival. Natural selection causes evolution
of a species as these beneficial traits become more common within a population.
Artificial Selection
Artificial selection occurs when humans select which plants or animals to breed to pass specific traits on to the next
generation. For example, a farmer may choose to breed only cows that produce the best milk and not breed cows
that produce less milk . Humans have also artificially bred dogs to create new breeds (Figure 7.21 ).
Artificial Selection Humans used artificial selection to create these different
breeds. Both dog breeds are descended
from the same wolves and their genes
are almost identical. Yet there is at least
one difference between their genes that
determine size.
Reproductive Isolation
There are two main ways that speciation happens naturally. Both processes create new species by isolating populations of the same species from each other. Organisms can be geographically isolated or isolated by a behavior. Over
a long period of time, usually thousands of years, each of the isolated populations evolves in a different direction.
How do you think scientists test whether two populations are separate species? They bring species from two populations back together again. If the two populations do not mate and produce fertile offspring, they are separate
Geographic Isolation
Allopatric speciation happens when groups from the same species are geographically isolated for long periods.
Imagine all the ways that plants or animals could be isolated from each other:
A mountain range.
A canyon.
Rivers, streams, or an ocean.
A desert.
Here are two examples of allopatric speciation:
• Darwin observed thirteen distinct finch species on the Galápagos Islands that had evolved from the same
ancestor. Different finch populations lived on separate islands with different environments. They evolved to
best adapt to those particular environments. Later, scientists were able to determine which finches had evolved
into distinct species by bringing members of each population together. The birds that could not mate were
separate species.
• When the Grand Canyon in Arizona formed, two populations of one squirrel species were separated by the
giant canyon, shown in Figure 7.22 and Figure 7.23 . After thousands of years of isolation from each other,
the squirrel populations on the northern wall of the canyon looked and behaved differently from those on the
southern wall. North rim squirrels have white tails and black bellies. Squirrels on the south rim have white
bellies and dark tails. They cannot mate with each other, so they are different species.
Isolation without Physical Separation
Sympatric speciation happens when groups from the same species stop mating because of something other than
physical separation, such as behavior. The separation may be caused by different mating seasons, for example.
Sympatric speciation is more difficult to identify.
Here are two examples of sympatric speciation:
• Some scientists suspect that two groups of orcas (killer whales) live in the same part of the Pacific Ocean part
of the year, but do not mate. The two groups hunt different prey species, eat different foods, sing different
songs, and have different social interactions (Figure 7.24 ).
• Two groups of Galápagos finch species lived in the same space, but each had their own distinct mating signals.
Members of each group selected mates according to different beak structures and bird calls. The behavioral
differences kept the groups separated until they formed different species.
Abert squirrel on the southern rim of the
Grand Canyon.
Kaibab squirrel found on northern rim of
the Grand Canyon.
Scientists that two types of orca whales
live in the same part of the Pacific
Ocean for part of the year but do not
Rates of Evolution
How fast is evolution? How long did it take for the giraffe to develop a long neck? How long did it take for the
Galápagos finches to evolve? How long did it take for whales to evolve from land mammals? These and other
questions about the rate of evolution are difficult to answer.
The rate of evolution depends on how much an organism’s genotype changes over a period of time. Evolution is
usually so gradual that we do not see the change for many, many generations.
Not all organisms evolve at the same rate. Humans took millions of years to evolve from a mammal that is now
extinct. It is very difficult to observe evolution in humans. However, there are organisms that are evolving so fast
that you can observe evolution! A human takes about 22 years to go through one generation. But some bacteria go
through over a thousand generations in less than two months. Since bacteria go through many generations in a few
days, we can actually trace their evolution as it is happening.
Evolutionary Trees
Charles Darwin came up with the idea of an evolutionary tree to represent the relationships between different species
and their common ancestors (Figure 7.25 ). The base of the tree represents the ancient ancestors of all life. The
separation into large branches shows where these original species evolved into different populations.
The branches keep splitting into smaller and smaller branches as species continue to evolve into more and more
species. Some species are represented by short twigs spurting out of the tree, then stopping. These are species that
went extinct before evolving into new species. Other “Trees of Life” have been created by other scientists (Figure
7.26 ).
An interactive Tree of Life can be found at
&#8220 Tree of Life&#8221 to represent how species evolve and diverge
into separate directions. Each point on
the tree where one branch splits off
from another represents the common
ancestor of the species on the separate
Lesson Summary
• Microevolution results from evolutionary changes that are small and do not lead to the creation of a new
• Macroevolution refers to large evolutionary changes that result in new species.
• The creation of a new species is called speciation.
• Allopatric speciation occurs when groups from the same species are geographically isolated physically for
long periods.
• Sympatric speciation occurs when groups from the same species stop interbreeding, because of something
other than physical separation, such as behavior.
• The rate of evolution is a measurement of the speed of evolution.
• Evolutionary trees are used to represent the relationships between different species and their common ancestors.
Review Questions
1. What is the difference between macroevolution and microevolution?
2. What do the branches on the Tree of Life represent?
3. Which organism has a faster rate of evolution: a human or a bacterium?
4. What are two possible reasons why organisms to evolve and adapt?
Apply Concepts
5. How do you know if two related organisms are members of the same species?
6. Why do the squirrels on opposite side of the Grand Canyon look different?
Scientists have drawn many different versions of the &#8220 Tree of
Life&#8221 to show different features of
evolution. This Tree of Life was made by
Ernst Haeckel in 1879.
7. How is artificial selection different from natural selection?
8. What, other than physical isolation, could cause a species to split into two different directions of evolution?
Critical Thinking
9. "We have no current evidence of evolution occurring." Is the above statement true or false? Why or why not?
Further Reading / Supplemental Links
Yeh, Jennifer, Modern Synthesi, (From Animal Sciences).
Darwin, Charles, Origin of the Specie, Broadview Press (Sixth Edition), 1859.
Ridley, Matt, The Red Queen: Sex and the Evolution of Human Nature, Perennial Books, 2003.
Ridley, Matt, Genome, Harper Collins, 2000.
Sagan, Carl, Cosmos, Edicions Universitat Barcelona, 2006.
Carroll, Sean B., The Making of the Fittest: DNA and the Ultimate Forensic Record of Evolution, Norton,
Dawkins, Richard, The Blind Watchmaker, W.W. Norton #38; Company, 1996.
Dawkins, Richard, The Selfish Gene, Oxford University Press, 1989.
Diamond, Jared, The Third Chimpanzee: The Evolution and Future of the Human Animal, HarperCollins,
Mayr, Ernst, What Evolution Is, Basic Books. 2001.
Zimmer, Carl, Smithsonian Intimate Guide to Human Origins, Smithsonian Press, 2008.
Points to Consider
• How old is the Earth?
• When did the first life forms develop?
• For how much of Earth’s history have humans existed?
History of Life on Earth
Lesson Objectives
• Explain how geologists and paleontologists use evidence to determine the history of Earth and life on Earth.
• Define the age of the earth as four to five billion years old.
• Explain how scientists need to know what the environment (what chemicals were around, the temperature,
etc.) was like on Earth billions of years ago to know how life formed.
Check Your Understanding
• What are fossils?
• How does the fossil record contribute to the evidence of evolution?
Cambrian explosion A sudden burst of evolution that may have been triggered by an environmental change(s);
made the environment more suitable for a wider variety of life forms; occurred during the Cambrian Period.
extinct Something that does not exist anymore; a group of organisms that has died out without leaving any living
geologic time scale A time scale used to describe when events happened in the history of Earth.
mass extinction An extinction when many species go extinct during a relatively short period of time.
stromolites Fossils made of algae and a kind of bacteria; some of the oldest fossils on Earth.
The Age of Earth
How old is Earth? How was it formed? How did life begin on Earth? These questions have fascinated scientists
for centuries. During the 1800s, geologists, paleontologists and naturalists found several forms of physical evidence
that confirmed that Earth is very old.
The evidence includes:
• Fossils of ancient sea life on dry land far from oceans. This supported the idea that the earth changed over
time and that some dry land today was once covered by oceans.
• The many layers of rock. When people realized that rock layers represent the order in which rocks and fossils
appeared, they were able to trace the history of Earth and life on Earth.
• Indications that volcanic eruptions, earthquakes and erosion that happened long ago shaped much of the earth’s
surface. This supported the idea of an older Earth.
Over 4 Billion Years
The earth is at least as old as its oldest rocks. The oldest rock minerals found on Earth so far are crystals that are at
least 4.404 billion years old. These tiny crystals were found in Australia. Likewise, Earth cannot be older than the
solar system. The oldest possible age of Earth is 4.57 billion years old, the age of the solar system. Therefore, the
age of Earth is between 4.4 and 4.57 billion years.
Origin of Life on Earth
There is good evidence that life has probably existed on Earth for most of Earth’s history. Fossils of blue-green algae
found in Australia are the oldest fossils of life forms on Earth. They are at least 3.5 billion years old (Figure 7.27 ).
Some of the oldest fossils on Earth are
made of algae and a kind of bacteria
found along the coast of Australia.
Life from Random Reactions
How did evolution begin? It started with the first signs of life. How did life begin 3.5 to 4 billion years ago? In order
to answer this question, scientists need to know what kinds of materials were available at that time. We know that
the ingredients for life were present at the beginning of Earth’s history. Scientists believe early Earth did not contain
oxygen gas, but did contain other gases, including:
Carbon dioxide
Carbon monoxide
Water vapor
Hydrogen sulfide
Where did these ingredients come from? Some chemicals were in water and volcanic gases. Other chemicals would
have come from meteorites in space. Energy to drive chemical reactions was provided by volcanic eruptions and
lightning. Today, we have evidence that life on Earth came from random reactions between chemical compounds,
which formed molecules. These molecules created proteins and nucleic acids (RNA or DNA), and then cells.
How long did it take to form the first life forms? As much as 1 billion years (Figure 7.28 ). Many scientists still
study the origin of the first life forms because there are many questions left unanswered, such as, "Did proteins or
nucleic acids develop first?"
Some clues to the origins of life on Earth
come from studying the early life forms
that developed in hot springs such as
the Grand Prismatic Spring at Yellowstone National Park. This spring is approximately 250 feet deep and 300 feet
Geologic Time Scale
Geologists and other earth scientists use geologic time scales to describe when events happened in the history of
Earth. The time scales can be used to show when both geologic events and events affecting plant and animal life
occurred. The geologic time scale in Figure 7.29 illustrates the timing of events like:
Volcanic eruptions
Major erosion
Meteorites hitting Earth
The first signs of life forms
Mass extinctions
Evolution of Major Life Forms
Life on Earth began about 3.5 to 4 billion years ago. The first life forms were single-cell organisms similar to
bacteria. The first multicellular organisms did not appear until about 610 million years ago. Many of the modern
types of organisms we know today evolved during the next ten million years, in an event called the Cambrian
explosion. This sudden burst of evolution may have been caused by some environmental changes that made the
environment more suitable for a wider variety of life forms.
The geologic time scale of Earthś past is organized according to events which took place during different periods
on the time scale. Geologic time is the same as the age of the earth between 4.404 and 4.57 billion years. Look
closely for such events as the extinction of dinosaurs and many marine animals.
Plants and fungi did not appear until roughly 500 million years ago. They were soon followed by arthropods (insects
and spiders). Next came the amphibians about 300 million years ago, followed by mammals around 200 million
years ago and birds around 100 million years ago.
Even though large life forms have been very successful on Earth, most of the life forms on Earth today are still
prokaryotes – small, single-celled organisms. Fossils indicate that many organisms that lived long ago are extinct.
Extinction of species is common; in fact, it is estimated that 99% of the species that have ever lived on Earth no
longer exist.
The basic timeline of a 4.6 billion year old Earth includes the following:
About 3.5 - 3.8 billion years of simple cells (prokaryotes).
3 billion years of photosynthesis.
2 billion years of complex cells (eukaryotes).
1 billion years of multicellular life.
600 million years of simple animals.
570 million years of arthropods (ancestors of insects, arachnids and crustaceans).
550 million years of complex animals.
500 million years of fish and proto-amphibians.
475 million years of land plants.
400 million years of insects and seeds.
360 million years of amphibians.
300 million years of reptiles.
200 million years of mammals.
150 million years of birds.
130 million years of flowers.
65 million years since the non-avian dinosaurs died out.
2.5 million years since the appearance of Homo.
200,000 years since the appearance of modern humans.
25,000 years since Neanderthals died out.
Mass Extinctions
An organism goes extinct when all of the members of a species do not exist anymore. Extinctions are part of natural
selection. Species often go extinct when their environment changes and they do not have the traits they need to
survive. Only those individuals with the traits needed to live in a changed environment survive (Figure 7.30 ).
Mass extinctions, such as the extinction of dinosaurs and many marine mammals, happened after major catastrophes
such as volcanic eruptions and earthquakes (Figure 7.31 ).
Since life began on Earth, there have been several major mass extinctions. If you look closely at the geological time
scale, you will find that at least five major massive extinctions have occurred in the past 540 million years. In each
mass extinction, over 50% of animal species died. The total number of mass extinctions could be as high as 20.
Two of the largest extinctions are described below:
• At the end of the Permian period about 99.5% of individual organisms went extinct! Up to 95% of marine
species perished, compared to “only” 70% of land species. Some scientists theorize that the extinction was
caused by the formation of Pangea, or one large continent made out of many smaller ones. One large continent
has a smaller shoreline than many small ones, so reducing the shoreline space may have caused so much marine
life to go extinct (Figure 7.32 ).
Humans have caused many extinctions by introducing species to new
places. For example many of New
Zealand&#8217 s birds have adapted to
nesting on the ground. This was possible because there were no land mammals in New Zealand until Europeans
arrived and brought cats fox and other
predators with them. Several of New
Zealand&#8217 s ground nesting birds
such as this flightless kiwi are now extinct or threatened because of these
The fossil of Tarbosaurus one of the
land dinosaurs that went extinct during
one of the mass extinctions.
The supercontinent Pangaea encompassed all of today&#8217 s continents
in a single land mass. This configuration limited shallow coastal areas which
harbor marine species and may have
contributed to the dramatic event which
ended the Permian - the most massive
extinction ever recorded.
• At the end of the Cretaceous period, or 65 million years ago, all dinosaurs (except those which led to birds)
went extinct (Figure 7.33 ). Some scientists believe a possible cause is a collision between the Earth and a
comet or asteroid. The collision could have caused tidal waves, changed the climate, and reduced sunlight by
10-20%. A decrease in photosynthesis would have resulted in less plant food, leading to the extinction of the
Evidence for the extinction of dinosaurs by asteroid includes a iridium-rich layer in the earth, dated at 65.5 million
years ago. Iridium is rare in the Earth’s crust, but common in comets and asteroids. Maybe the asteroid that hit the
earth left the iridium behind.
After each mass extinction, new species develop to fill the habitats where old species lived. This is well documented
in the fossil record (Figure 7.34 ).
Lesson Summary
• During the 1800s, geologists, paleontologists and naturalists found several forms of physical evidence that
confirmed that the earth is over 4 billion years old.
• Geologists and other earth scientists use geologic time scales to describe when events occurred throughout the
history of Earth.
• The first life forms were single-celled organisms, prokaryotic organisms, similar to bacteria.
• Mass extinctions, such as the extinction of dinosaurs and many marine mammals, happened after major catastrophes such as volcanic eruptions and major earthquakes changed the environment.
Review Questions
1. How do scientists determine the age of a rock or fossil today?
2. How do we know the maximum and minimum possible age of the Earth?
3. How long ago did life start on Earth?
4. When did mammals first appear on Earth?
The fossil record demonstrates the
presence of dinosaurs which went extinct over 65 million years ago.
Mammals and birds quickly invaded ecological niches formerly occupied by the dinosaurs. Mammals included
monotremes A marsupials and hoofed placentals B. Modern sharks C patrolled the seas. Birds included the
giant flightless Gastornis D.
5. What kinds of events are recorded on a geological time scale?
Apply Concepts
6. Why is it difficult to determine how life started on Earth?
7. Why are extinctions a part of natural selection?
Think Critically
8. In order to develop the best theory for the extinction of the dinosaurs, what other information might be useful?
9. You are a scientist investigating the origin of life on earth? Ask three questions that will help you complete your
Further Reading / Supplemental Links
Stein, Sara, The Evolution Book, Workman, N.Y., 1986.
Yeh, Jennifer, Modern Synthesis, (From Animal Sciences).
Darwin, Charles, Origin of the Species, Broadview Press (Sixth Edition), 1859.
Ridley, Matt, The Red Queen: Sex and the Evolution of Human Nature, Perennial Books, 2003.
Ridley, Matt, Genome, Harper Collins, 2000.
Sagan, Carl, Cosmos, Edicions Universitat Barcelona, 2006.
Carroll, Sean B., The Making of the Fittest: DNA and the Ultimate Forensic Record of Evolution, Norton,
Dawkins, Richard, The Blind Watchmaker, W.W. Norton #38; Company, 1996.
Dawkins, Richard, The Selfish Ge Oxford University Press, 1989.
Diamond, Jared, The Third Chimpanzee: The Evolution and Future of the Human Animal, HarperCollins,
Mayr, Ernst, What Evolution Is, Basic Books, 2001.
Zimmer, Carl, Smithsonian Intimate Guide to Human Origins, Smithsonian Press, 2008.
Points to Consider
• What are prokaryotic organisms?
• Compare and contrast prokaryotes and eukaryotes.
MS Prokaryotes
The above image shows bacteria dyed with a fluorescent color. They look just like little cells. Well, that’s exactly
what they are.
Bacteria are prokaryotic organisms. About 3.5 billion years ago, long before the first plants, people, or other animals
appeared, prokaryotes were the first life forms on Earth. For at least a billion years, prokaryotes ruled the Earth as
the only existing organisms.
What do you think of when you think of bacteria? Germs? Diseases? Bacteria can be harmful, but they can also
help you. How do you think bacteria can help humans and other organisms?
Did you know that bacteria are not the only type of prokaryote? There is another type, called archaea, which we will
explore in addition to the questions asked above.
Lesson Objectives
Describe the cellular features of bacteria.
Explain the ways in which bacteria can obtain energy.
Explain how bacteria reproduce themselves.
Identify some ways in which bacteria can be helpful.
Identify some ways in which bacteria can be harmful.
Check Your Understanding
• How do prokaryotic and eukaryotic cells differ?
• What are some components of all cells, including bacteria?
bacilli Rod-shaped bacteria or archaea.
chemotroph Organism that obtains energy by oxidizing compounds in their environment.
cocci Sphere-shaped bacteria or archaea.
conjugation The transfer of genetic material between two bacteria.
cyanobacteria Photosynthetic bacteria.
decomposer Organism that break down wastes and dead organisms and recycle their nutrients back into the environment.
flagella Some bacteria also have tail-like structures called flagella.
nucleoid The prokaryotic DNA consisting of a condensed single chromosome.
peptidoglycan Complex molecule consisting of sugars and amino acids that makes up the bacterial cell wall.
spirilli Spiral-shaped bacteria or archaea.
transduction Transfer of DNA between two bacteria; occurs with the aid of a virus (bacteriophage).
Characteristics of Bacteria
Even though life is much more diverse on Earth today, bacteria (singular: bacterium) are still the most abundant
organisms on Earth. Recall that prokaryotes are single-celled organisms that lack a nucleus, and that the prokaryotes
include bacteria and archaea.
Size and Shape
Bacteria are so small that they can only be seen with a microscope. When viewed under the microscope, they have
three distinct shapes (Figure" 8.1 , Figure 8.2 , and Figure 8.3 ). Bacteria can be classified by their shape:
a. Bacilli are rod-shaped.
b. Cocci are sphere-shaped.
c. Spirilli are spiral-shaped.
Bacteria come in many different shapes.
Some of the most common shapes are
bacilli rods cocci spheres and spirilli
spirals. Bacteria can be identified and
classified by their shape.
Escherichia coli is an example of bacteria that are rod-shaped or bacilli.
Staphylococcus aureus is an example
of bacteria that are sphere-shaped or
The Cell Wall
Bacteria are surrounded by a cell wall consisting of peptidoglycan, a complex molecule consisting of sugars and
amino acids. The cell wall is important for protecting bacteria. The cell wall is so important that some antibiotics,
such as penicillin, kill bacteria by preventing the cell wall from forming.
Another type of bacteria, called parasitic bacteria, depends on a host organism for energy and nutrients. If the host
starts attacking the bacteria, the bacteria release a layer of slime that surrounds the cell wall for an extra layer of
Differences between Eukaryotes and Prokaryotes
Recall that all prokaryotes, including bacteria, lack many of the things that eukaryotes contain, such as membranebound organelles (like mitochondria or chloroplasts) or a nucleus (Figure 8.4 ).
Similarities to Eukaryotes
Like eukaryotic cells, prokaryotic cells do have:
Cytoplasm, the fluid inside the cell.
A plasma membrane, which acts as another barrier.
Ribosomes, where proteins are assembled.
DNA, contained in a large circular strand, forming a single chromosome, that is compacted into a structure
called the nucleoid. Many bacteria also have additional small rings of DNA known as plasmids.
The structure of a bacterial cell is distinctive from a eukaryotic cell because
of features such as an outer cell wall
and the circular DNA of the nucleoid
and the lack of membrane-bound organelles.
Some bacteria also have tail-like structures called flagella (Figure 8.5 ). Flagella help bacteria move. As the flagella
rotate, they spin the bacteria and propel them forward.
The flagella facilitate movement in bacteria. Bacteria may have one two or
many flagella - or none at all.
Obtaining Food and Energy
Bacteria obtain energy and nutrients in a variety of different ways:
• Bacteria known as decomposers break down wastes and dead organisms into smaller molecules to get the
energy they need to survive.
• Photosynthetic bacteria use the energy of the sun, together with carbon dioxide, to make their own food.
Briefly, in the presence of sunlight, carbon dioxide and water is turned into glucose and oxygen. The glucose
is then turned into usable energy. Glucose is like the "food" of the bacteria. An example of photosynthetic
bacteria is cyanobacteria, as seen in Figure 8.6 .
• Bacteria can also be chemotrophs. Chemotrophs obtain energy by breaking down chemical compounds in
their environment, such as nitrogen-containing ammonia. They do not use the energy from the sun. Nitrogen
cannot be made by living organisms, so it must be continually recycled. The bacteria help cycle the nitrogen
through the environment for other living things to use. Organisms need nitrogen to make organic compounds,
such as DNA.
• Some bacteria depend on other organisms for survival. For example, mutualistic bacteria live in nutrient-rich
part of the roots of legumes, such as pea plants. The bacteria turn nitrogen-containing molecules into nitrogen
that the plant can use. In this relationship, both the bacteria and the plant benefit.
• Other bacteria are parasitic and can cause illness. In a parasitic relationship, the bacteria benefit and the other
organism is harmed. Harmful bacteria will be discussed later in the lesson.
Reproduction in Bacteria
Bacteria reproduce through a process called binary fission. During binary fission, the chromosome copies itself,
forming two genetically identical copies. Then, the cell enlarges and divides into two new daughter cells. The two
daughter cells are identical to the parent cell (Figure 8.7 ). Binary fission can happen very rapidly. Some species of
bacteria can double their population in less than ten minutes! (Figure ?? )
Cyanobacteria are photosynthetic bacteria. These bacteria carry out all the reactions of photosynthesis within the cell
membrane and in the cytoplasm they
do not need chloroplasts.
Sexual reproduction does not occur in bacteria. But not all new bacteria are clones. This is because bacteria can still
combine and exchange DNA. This exchange occurs in three different ways:
a. Conjugation: In conjugation, DNA passes through an extension on the surface of one bacterium and travels
to another bacterium.
b. Transformation: In transformation, bacteria pick up pieces of DNA from their environment.
c. Transduction: In transduction, viruses that infect bacteria carry DNA from one bacterium to another.
Helpful Bacteria
Bacteria are helpful to humans and to other living things because they can:
Recycle essential nutrients in the soil.
Aid in animal digestion.
Produce food for consumption.
Produce chemicals used in medicines.
Bacteria are important because many bacteria are decomposers. They break down dead materials and waste products
and recycle nutrients back into the environment. This recycling of nutrients, such as nitrogen, is essential for living
organisms. Organisms cannot produce nutrients, so they must come from other sources.
We get nutrients from the food we eat; plants get them from the soil. How do these nutrients get into the soil? One
way is from the actions of decomposers. Without decomposers, we would eventually run out of the materials we
need to survive. We also depend on bacteria to decompose our wastes in sewage treatment plants.
Bacteria cells reproduce by binary fission resulting in two daughter cells identical to the parent cell.
Bacteria also help you digest your food. Several species of bacteria, such as E. coli, are found in your digestive tract.
In fact, bacteria cells outnumber your own cells in your gut!
Food Production
Bacteria are involved in producing some foods. Yogurt is made by using bacteria to ferment milk. Cheese can also
be made from milk with the help of bacteria (Figure 8.8 ). Fermenting cabbage with bacteria produces sauerkraut.
Yogurt is made from milk fermented
with bacteria. The bacteria ingest natural milk sugars and release lactic acid
as a waste product which causes proteins in the milk to form into a solid mass
which becomes the yogurt.
In the laboratory, bacteria can be changed to provide us with a variety of useful materials. Bacteria can be used as
tiny factories to produce desired chemicals and medicines. For example, insulin, which is necessary to treat people
with diabetes, can be produced using bacteria.
Through the process of transformation, the human gene for insulin is placed into bacteria. The bacteria then use that
gene to make a protein. The protein can be separated from the bacteria, and then used to treat patients. The mass
production of insulin by bacteria made this medicine much more affordable.
Harmful Bacteria
There are also ways that bacteria can be harmful to humans and other animals.
Bacteria are responsible for many types of human illness, including:
Strep throat.
Lyme disease.
The Black Death, which killed at least one third of Europe’s population in the 1300s, is believed to have been caused
by the bacterium Yersinia pestis.
Food Contamination
Bacterial contamination can also lead to outbreaks of food poisoning. Raw eggs and undercooked meats can contain
bacteria that can cause digestive problems. Foodborne infection can be prevented by cooking meat thoroughly and
washing surfaces that have been in contact with raw meat. Washing of hands before and after handling food also
helps stop contamination.
Some bacteria also have the potential to be used as biological weapons by terrorists. An example is anthrax, a
disease caused by the bacterium Bacillus anthracis. Since inhaling the spores of this bacterium can lead to a deadly
infection, it is a dangerous weapon. In 2001, an act of terrorism in the United States involved B. anthracis spores
sent in letters through the mail.
Lesson Summary
• Bacteria contain a cell wall containing peptidoglycan and a single chromosome contained in the nucleoid.
• Bacteria can obtain energy through several means including photosynthesis, decomposition, and parasitism,
symbiosis, and chemosynthesis.
• Bacteria reproduce through binary fission.
• Bacteria are important decomposers in the environment and aid in digestion.
• Some bacteria can be harmful when they contribute to disease, food poisoning, or biological warfare.
Review Questions
1. What are prokaryotes?
2. What are the possible shapes that bacteria can have?
3. What is the purpose of the flagella?
4. What is a chemotroph?
Apply Concepts
5. How is the DNA in prokaryotes different from the DNA in eukaryotes?
6. How do bacteria reproduce without having sex?
7. What are the ways bacteria can go through genetic recombination?
8. How are cyanobacteria similar to plants?
9. How are bacteria important in nature?
10. How can you avoid becoming sick from bacteria that cause food poisoning?
Think Critically
11. If a species of bacteria lives in the roots of a plant and supplies the plant with nitrogen, is this a parasitic bacteria?
Explain why or why not.
12. How might you genetically modify bacteria so that they produce a chemical they do not normally produce?
Further Reading / Supplemental Links
Points to Consider
• In the next section we will discuss the Kingdom Archaea. “Archae” shares the same root word as “archives”
and “archaic,” so what do you think it means?
• What do you think the earliest life forms on Earth looked like?
• How do you think these early life forms obtained energy?
Lesson Objectives
Identify the differences between archaea and bacteria.
Explain how the archaea can obtain energy.
Explain how the archaea reproduce.
Discuss the unique habitats of the archaea.
Check Your Understanding
• What are the three shapes of bacteria?
• How do bacteria reproduce?
• How can bacteria be harmful?
halophiles Organisms that live and thrive in very salty environments.
methanogens Organisms that live in swamps or in the guts of cows and termites and release methane gas.
thermophiles Organisms that live in very hot environments, such as near volcanoes and in geysers.
What are Archaea?
For many years, archaea were classified as bacteria. However, when scientists compared the DNA of the two
prokaryotes, they found that there were two distinct types of prokaryotes, which they named archaea and bacteria.
Even though the two groups might seem similar, archaea have many features that distinguish them from bacteria:
a. The cell walls of archaea are distinct from those of bacteria. In most archaea, the cell wall is assembled from
proteins, providing both chemical and physical protection. In contrast to bacteria, most archaea do not have
peptidoglycan in their cell walls.
b. The plasma membranes of the archaea also are made up of lipids that are distinct from those in bacteria.
c. The ribosomal proteins of the archaea are similar to those in eukaryotic cells, not those in bacteria.
Although archaea and bacteria share some fundamental differences, they are still similar in many ways:
a. They both are single-celled, microscopic organisms that can come in a variety of shapes (Figure 8.9 ).
b. Both archaea and bacteria have a single circular chromosome of DNA and lack membrane-bound organelles.
c. Like bacteria, archaea can have flagella to assist with movement.
Archaea shapes can vary widely but
some are bacilli or rod-shaped.
Obtaining Food and Energy
Most archaea are chemotrophs and derive their energy and nutrients from breaking down molecules in their environment. A few species of archaea are photosynthetic and capture the energy of sunlight. Unlike bacteria, which can
be parasites and are known to cause a variety of diseases, there are no known archaea that act as parasites. Some
archaea do live within other organisms. But these archea form mutualistic relationships with their host, where both
the archaea and the host benefit. In other words, they assist the host in some way, for example by helping to digest
Like bacteria, reproduction in archaea is asexual. Archaea can reproduce through binary fission, where a parent
cell divides into two genetically identical daughter cells. Archaea can also reproduce asexually through budding and
fragmentation, where pieces of the cell break off and form a new cell, also producing genetically identical organisms.
Types of Archaea
The first archaea described were unique in that they could survive in extremely harsh environments where no other
organisms could survive.
The halophiles, which means "salt-loving," live in environments with high levels of salt (Figure 8.10 ). They have
been identified in the Great Salt Lake in Utah and in the Dead Sea between Israel and Jordan, which have salt
concentrations several times that of the oceans.
shown here require high salt concentrations.
The thermophiles live in extremely hot environments (Figure 8.11 ). For example, they can grow in hot springs,
geysers, and near volcanoes. Unlike other organisms, they can thrive in temperatures near 100ºC, the boiling point
of water!
Methanogens can also live in some strange places, such as swamps, and inside the guts of cows and termites. They
help these animals break down cellulose, a tough carbohydrate made by plants (Figure 8.12 ). This is an example of
a mutualistic relationship. Methanogens are named for their waste product, a gas called methane.
Although archaea are known for living in unusual environments, like the Dead Sea, inside hot springs, and in the guts
of cows, they also live in more common environments. For example, new research shows that archaea are abundant
Thermophiles can thrive in hot springs
and geysers such as this one the Excelsior Geyser in the Midway Geyser Basin
of Yellowstone National Park Wyoming.
Cows are able to digest grass with the
help of the methanogens in their gut.
in the soil and among the plankton in the ocean (Figure 8.13 . Therefore, scientists are just beginning to discover
some of the important roles that archaea have in the environment.
Thermococcus gammatolerans are another type of archaea.
Lesson Summary
Archaea are prokaryotes that differ from bacteria somewhat in their DNA and biochemistry.
Most archaea are chemotrophs, but some are photosynthetic or form mutualistic relationships.
Archaea reproduce asexually through binary fission, fragmentation, or budding.
Archaea are known for living in extreme environments.
Review Questions
1. What are the two types of prokaryotes?
2. How are the cell walls of archaea different from those of bacteria?
3. How do archaea obtain energy?
4. How do archaea reproduce?
5. Where do halophiles live?
6. Where do thermophiles live?
Apply Concepts
7. How could you tell the difference between archaea and bacteria?
8. If an organism is classified as a methanogen, what does this mean?
9. What is an example of a mutualistic relationship between archaea and another organism?
Think Critically
10. A teacher tells you she wants you to do a project on parasitic archaea. Why will you be unable to complete the
11. You find bacteria at the bottom of the Great Salt Lake, and another scientist calls them methanogens. Explain
why that scientist is incorrect.
Further Reading / Supplemental Links
Points to Consider
In the next chapter we will move on to the protists and fungi. How do you think they are different from archaea and
• Can you think of some ways that fungi can be helpful?
• Can you think of some ways that fungi can be harmful?
9.2 F UNGI
MS Protists and Fungi
What does the above image look like to you? A bacteria? An animal? A plant? Actually, it is not found in any of
those categories. The above organism is called a "protist."
Protists are a unique category of organisms because they are very different when compared to each other, but they
can be very similar to plants, animals, and fungi.
What are fungi? They are another kingdom of organisms that are not related to protists, but are equally interesting.
There are estimated to be 1.5 million species of fungi, although only 5% of them are classified.
Lesson Objectives
• Explain why protists cannot be classified as plants, animals, or fungi.
• List the similarities that exist between most protists.
• Identify the three subdivisions of the organisms in the kingdom Protista.
Check Your Understanding
• What are some basic differences between a eukaryotic cell and a prokaryotic cell?
• List some characteristics that all cells have.
autotroph Organism that produces complex organic compounds from simple inorganic molecules using a source
of energy such as sunlight.
cilia Finger-like projects from the cells; can be found from the cells of mucous membranes.
filter-feeder An organism that feeds by filtering organic matter out of water.
heterotroph Organism which obtains carbon from outside sources.
protist Eukaryotic organism that belongs to the kingdom Protista; not a plant, animal or fung
protozoa Animal-like protists.
pseudopodia A moving fake foot; the cell surface extends out a membrane and the force of this membrane propels
the cell forward.
What are Protists?
Protists are eukaryotes, and most are single-celled. You can think about protists as all eukaryotic organisms that are
neither animals, nor plants, nor fungi.
Even among themselves, they have very little in common. Although theses organisms were put in the category
Protista by Ernst Haeckel in 1866, the Kingdom Protista was not an accepted classification in the scientific world
until the 1960s. These unique organisms can be so different from each other that sometimes Protista is called the
“junk drawer kingdom.” This kingdom contains the eukaryotes that cannot be put into any other kingdom.
Unicellular or Multicellular?
Most protists, such as the ones shown in Figure 9.1 , are so small that they can be seen only with a microscope.
Protists are mostly unicellular (one-celled) eukaryotes that exist as independent cells. A few protists are multicellular
(many-celled) and surprisingly large. These protists do not, however, show cellular specialization or differentiation
into tissues. For example, kelp is a multicellular protist and can be over 100-meters long with cells that perform
mostly the same jobs.
Characteristics of Protists
A few characteristics are common between protists:
They are eukaryotic, which means they have a nucleus.
Most have mitochondria.
They can be parasites.
They all prefer aquatic or moist environments.
For classification, the protists are divided into three groups:
a. Animal-like protists
b. Plant-like protists
c. Fungi-like protists.
But remember, protists are not animals, nor plants, nor fungi (Figure 9.2 ).
Classification of Protists
As there are many different types of protists, the classification of protists can be difficult. Recently, scientists
confirmed that the protists are related by analyzing their DNA. Protists with more common DNA sequences are
more closely related to each other then those with fewer common DNA sequences.
Find information on different types of protists here:
How Protists Obtain Food
The cells of protists need to perform all of the functions that other cells do, such as grow and reproduce, maintain
homeostasis, and obtain energy. They also need to obtain food to provide the energy to perform these functions.
For such simple organisms, protists get their food in a complicated process. Although there are many photosynthetic
protists, such as algae, that get their energy from sunlight, many others must "swallow" their food through a process
called endocytosis. Endocytosis happens when a cell takes in substances through its membrane. The process is
described below:
The protist wraps its cell wall and cell membrane around its prey, which is usually bacteria.
It creates a food vacuole, a sort of "food storage compartment," around the bacteria.
The protist produces toxins which paralyze its prey.
Once paralyzed, the food material moves through the vacuole and into the cytoplasm of the protist.
Other protists are parasitic and absorb nutrients meant for their host, harming the host in the process.
Protists come in many different shapes.
This slime mold is a protist. Slime molds
had previously been classified as fungi
but are now placed in the Kingdom Protista. Slime molds live on decaying plant
life and in the soil.
Animal-like Protists
Animal-like, plant-like, and fungi-like protists are different from each other mainly because they have different ways
of getting carbon. Carbon is important in the formation of organic compounds like carbohydrates, lipids, proteins,
and nucleic acids. You get it from eating, as do other animals.
Animal-like protists are called protozoa. Protozoa are single-celled eukaryotes that share certain traits with organisms in the animal kingdom. Like animals, they can move, and they they get their carbon from outside sources. They
are heterotrophs, which means they eat things outside of themselves instead of producing their own food.
Animal-like protists are very small, measuring only about 0.01–0.5mm. Animal-like protists include the zooflagellates, ciliates, and the sporozoans (Figure 9.3 ).
Euglena are animal-like protists. Over
1000 species of Euglena exist. They
are used in industry in the treatment of
Some animal-like protists literally “eat with their tails.” The tail of a protist is a flagellum. These protists are called
flagellates. Flagellates are filter-feeders. They acquire oxygen and nitrogen by constantly whipping the flagellum
back and forth, a process called filter-feeding. The whipping of the flagellum creates a current that brings food into
the protist. Recall that prokaryotes can also have flagella (the plural of flagellum).
Different Kinds of Animal-like Protists
Are there different types of animal-like protists? Yes. They are different because they move in different ways.
• Flagellates have long flagella, or tails. Flagella rotate in a propeller-like fashion. An example of a flagellate is
the Trypanosoma, which causes African sleeping sickness.
• Other protists have what are called transient pseudopodia, which are like temporary feet. The cell surface
extends out a membrane, and the force of this membrane moves the cell forward. An example of a protist with
a pseudopod is the amoeba.
• Another way protists move is by the movement of cilia. Cilia are thin, very small tail-like projections that
extend outward from the cell body. Cilia beat back and forth, moving the protist along. The paramecium has
cilia that propel it.
• A few protists are do not move at all, such as the toxoplasma. These protists form spores that become new
protists, and are known as sporozoa.
Plant-like Protists
Plant-like protists are autotrophs. This means that they produce their own food. They perform photosynthesis to
produce sugar by using carbon dioxide and the energy from sunlight, just like plants. Plant-like protists live in soil, in
seawater, on the outer covering of plants, and in ponds and lakes (Figure 9.4 ). Protists like these can be unicellular
or multicellular. Some protists, such as kelp, live in huge colonies in the ocean.
Plant-like protists are essential to the environment because they produce oxygen through photosynthesis, which
helps other organisms, like animals, survive.
Plant-like protists are classified into a number of basic groups (Table 9.1 ).
TABLE 9.1: Plantlike Protists
green algae - related to
higher plants
red algae
brown algae
diatoms, golden-brown algae, yellow-green algae
Number (approximate)
Red algae are a very large group
5 000&#8211 6 000
are mostly multicellular live in the
ocean. Many red algae are seaweeds
and help create coral reefs.
Fungus-like Protists
Fungus-like protists are heterotrophs that have cell walls and reproduce by forming spores (see Lesson 9.2 for more
information about spores). Fungus-like protists usually do not move, but some develop movement at some point in
their lives.
There are essentially three types of fungus-like protists (see Table 9.2 ):
a. Water molds.
b. Downy mildews.
c. Slime molds.
Slime molds represent the characteristics of the fungus-like protists. Most slime molds measure about one or two
centimeters, but a few slime molds are as big as several meters. They often have bright colors, such as a vibrant
yellow. Others are brown or white.
Stemonitis is a kind of slime mold which forms small brown bunches on the outside of rotting logs. Physarum
polycephalum lives inside rotting logs and is a gooey mesh of yellow "threads" that are a several centimeters long.
Fuligo, sometimes called “vomit mold,” is a yellow slime mold found in decaying wood.
TABLE 9.2: Fungus-like Protists
omycetes: water molds
(Figure 9.5 )
Source of Carbon
decomposed remains, parasites of plants and animals
most live in water
Mycetozoa: slime molds
(Figure 9.6 )
dispose of dead plant material, feed on bacteria
common in soil, on lawns,
and in the forest commonly on deciduous logs
Causes a range of diseases in plants; common
problem in greenhouses
where the organism kills
new seedlings (plants
from seeds); includes the
downy mildews, which
are easily identifiable by
the appearance of white
"mildew" on leaves.
Includes the cellular slime
mold, which involves numerous individual cells
attached to each other,
forming one large "supercell," essentially a bag
of cytoplasm containing
thousands of individual
nuclei. The plasmodial
slime molds spend most
of their lives as individual
cells, but when a chemical signal is released, they
form a cluster that acts as
one organism.
An example of a slime mold.
An aquatic insect nymph attacked by
water mold.
Importance of Protists
Humans could not live on Earth if it were not for protists. Why? Protists produce almost one-half of the oxygen on
the planet, decompose and recycle nutrients that humans need to live, and make up a huge part of the food chain.
Humans use protists for many other reasons:
• Many protists are also commonly used in medical research. For example, medicines made from protists are
used in treatment of high blood pressure, digestion problems, ulcers, and arthritis.
• Other protists are used in scientific studies. For example, slime molds are used to analyze the chemical signals
used in cells.
• Protists are also valuable in industry. Look on the back of a milk carton. You will most likely see carrageenan,
which is extracted from red algae. This is used to make puddings and ice cream solid. Chemicals from other
kinds of algae are used to produce many kinds of plastics.
Lesson Summary
• Protists are highly diverse organisms that belong to the Kingdom Protista.
• Protists are divided into three subgroups: animal-like protists, plant-like protists and fungus-like protists.
• Animal-like protists are unicellular eukaryotes that share certain traits with animals, such as mobility and
• Plant-like protists are unicellular or multicellular autotrophs that live in soil, in seawater, on the outer covering
of plants, and in ponds and lakes.
• Fungus-like protists, such as water molds, downy mildews, and slime molds, are heterotrophs that reproduce
by forming spores.
Review Questions
1. List the characteristics that all protists share.
2. List two ways that protists obtain food.
3. Describe the characteristics of an animal-like protist.
4. Describe the characteristics of a plant-like protist.
5. Describe the characteristics of a fungi-like protist.
6. Name three kinds of fungi-like protists.
Apply Concepts
7. Explain why protists are important to life on Earth.
8. You find a protist that is a heterotroph and lives in the ocean. Is this protist most similar to a plant, animal, or
fungus? Why or why not?
Critical Thinking
9. Imagine that you are a scientist delivering a paper called Protists: the Junk-Drawer Kingdom. Explain your
reasoning for this title?
Further Reading / Supplemental Links
• King, Katie and Ball, Jacqueline, Protists and Fungi. 2003 Gareth Stevens Publishing.
• Marguilis, L., Corliss, J.O., Melkonian, M.,and Chapman, D.J. (Editors) 1990. Handbook of Protoctista.
Jones and Bartlett, Boston.
• Jahn,T.L., Bovee, E.C. #38; Jahn, F.F. 1979 How to Know the Protozoa. 2nd ed. Wm. C. Brown Publishers,
Div. of McGraw Hill, Dubuque, Iowa.
• Patterson, D.J. 1996. Free-Living Freshwater Protozoa: A Colour Guide. John Wiley #38; Sons, NY.
• Streble H., Krauter D. 1988. Life in a waterdrop. Microscopic freshwater flora and fauna. An identification
Points to Consider
• Fungi comprise one of the eukaryotic kingdoms. Think about what might distinguish a fungi-like protist from
a true fungus?
• Given the vast differences between the protists discussed in this lesson, think about the possibilities of dividing
this kingdom into additional kingdoms. How might that division be accomplished? Is that a good idea or would
it just lead to confusion?
Lesson Objectives
• Describe the characteristics of fungi.
• Identify structures that distinguish fungi from plants and animals.
• Explain how fungi can be used in industry.
Check Your Understanding
• What is a significant difference between a protist and other eukaryotic organisms?
• What are some of the distinguishing characteristics of fungus-like protists?
budding Asexual reproduction in which part of the body of a fungus, for example, grows and breaks off, eventually
becoming a new organism.
chitin A nitrogen-containing material found in the cell wall of fungi; also found in the shells of animals such as
beetles and lobsters.
fruiting body Specialized structure used in sexual reproduction; part of the fungus that produces the spores.
hyphae Thread-like structures which interconnect and bunch up into mycelium; helps bring food, such as a worm,
inside the fungus.
mycelial fragmentation Asexual reproduction involving splitting off of the mycelia; a fragmented piece of mycelia
can eventually produce a new colony of fungi.
mycelium Help the fungi absorb nutrients from living hosts; composed of hyphae.
mycorrhizal symbiosis A relationship between fungi and the roots of plants where both benefit; the plant provides
sugar to the fungus; the fungi provides minerals and water to the roots of the plant.
parasite The organism that benefits in a relationship between two organisms in which one is harmed.
spore The basic reproductive unit of fungi.
What are Fungi?
Ever notice blue-green mold growing on a loaf of bread? Do you like your pizza with mushrooms? Has a physician
ever prescribed an antibiotic for you?
If so, then you have encountered fungi. Fungi are organisms that belong to the Kingdom Fungi (Figure 9.7 ). Our
environment needs fungi. Fungi help decompose matter and make nutritious food for other organisms. Fungi are all
around us and are useful in many ways.
These many different kinds of organisms that demonstrate the huge diversity within the Kingdom Fungi.
If you had to guess, would you say a fungus is a plant or animal? Scientists used to debate about which kingdom to
place fungi in. Finally they decided that fungi were plants. But they were wrong. Now, scientists know that fungi
are not plants at all. Fungi are very different from plants.
The main difference between plants and fungi is how they obtain energy. Plants are autotrophs, meaning that they
make their own "food" using the energy from sunlight. Fungi are heterotrophs, which means that they obtain their
"food" from outside of themselves. In other words, they must "eat" their food like animals and many bacteria do.
Yeasts, molds, and mushrooms are all different kinds of fungi (Figure 9.8 ). There may be as many as 1.5 million
species of fungi. You can easily see bread mold and mushrooms without a microscope, but most fungi you cannot
see. Fungi are either too small to be seen without a microscope, or they live where you cannot see them easily - deep
in the soil, under decaying logs, or inside plants or animals. Some fungi even live in or on top of other fungi.
Fungi and Symbiotic Relationships
If it were not for fungi, many plants would go hungry. In the soil, fungi grow closely around the roots of plants, and
they begin to help each other. This form of helping each other out is called mycorrhizal symbiosis. Mycorrhizal
means “roots” and symbiosis means “relationship” between organisms (Figure 9.9 ).
As the plants and fungi form the close relationship, the plant and the fungus “feed” one another. The plant provides
9.2. FUNGI
The blue in this blue cheese is actually
glucose and sucrose to the fungus that the plant makes through photosynthesis, which the fungus cannot do. The
fungi then provides minerals and water to the roots of the plant.
This mushroom and tree live in symbiosis with each other.
Have you ever seen an organism called a lichen? Lichens are crusty, hard growths that you might find on trees, logs,
walls, and rocks. Although lichens may not be the prettiest organisms in nature, they are unique. A lichen is really
two organisms that live very closely together: a fungus and a bacteria or algae. The cells from the algae or bacteria
live inside the fungus. Each organism provides nutrients for the other.
What is it called when two organisms live close together and form a relationship? Symbiosis. A lichen is the result
of symbiosis between a fungus and another organism. Because this relationship helps both organisms, it is called a
mutualistic relationship.
Fungi and Insects
Many insects have a symbiotic relationship with certain types of fungi:
• Ants and termites grow fungi in underground “fungus gardens” that they create. When the ants or termites
have eaten a big meal of wood or leaves, they also eat some fungi from their gardens. The fungi help them
digest the wood or leaves.
• Ambrosia beetles live in the bark of trees. Like ants and termites, they grow fungi inside the bark of trees and
use it to help digest their food.
Fungi as Parasites
Although lots of symbiotic relationships help both organisms, sometimes one of the organisms is harmed. When
that happens, the organism that benefits and is not harmed is called a parasite.
Examples of parasitic fungi include the following:
• Beginning in 1950, Dutch Elm trees in the United States began to die. Since then much of the species has
been eliminated. The disease was caused by a fungus that acted as a parasite. The fungus that killed the trees
was carried by beetles to the trees. The tree tried to stop the growth of the fungus by blocking its own ability
to gain water. However, without water the tree soon died.
• Some parasitic fungi cause human diseases such as athlete’s foot and ringworm. These fungi feed on the outer
layer of warm, moist skin.
Fungi as Predators
Fungi growing on a tree trunk does not seem very dangerous. But some fungi are actually hunters. For example,
some fungi trap nematodes. A nematode is a worm that some fungi like to eat. These hungry fungi live deep in the
soil where they set traps for unsuspecting nematodes by making a circle with their hyphae. Hyphae are like arms
and legs. They look like cobwebs and can be sticky. Fungi set out circular rings of hyphae with a lure inside, which
brings the nematode inside the fungus (Figure 9.10 ).
Fungi are Good Eaters
Fungi can grow fast because they are such good eaters. Fungi have lots of surface area and this large surface
area “eats.” Surface area is how much exposed area an organism has compared to their overall volume. Most of a
mushroom’s surface area is actually underground.
These are the steps involved in fungi eating:
a. Fungi squirt special enzymes into their environment.
b. The enzymes help digest large organic molecules, similar to cutting up your food before you eat.
c. Cells of the fungi then absorb the broken-down nutrients.
Why do you think a large surface area allows fungi to obtain more nutrients?
Fungi Body Parts
The most important body parts of a fungi include:
a. Cell wall: A layer around the cell membrane of fungi cells, similar to that found in plant cells.
9.2. FUNGI
Hyphae are the cobweb-like arms and
legs of fungi.
b. Hyphae: These are thread-like structures which interconnect and bunch up into a mycelium. Ever see mold
on a damp wall or on old bread? The things that you are seeing are really mycelia. The hyphae and mycelia
help the fungi absorb nutrients from other organisms.
c. Specialized structures for reproduction: One example is a fruiting body. A mushroom is a fruiting body,
which is the part of the fungus that produces spores (Figure 9.11 ). Those spores, discussed in the next
section, are the basic reproductive units of fungi.
A mushroom is a fruiting body.
Fungi Reproduction
Reproduction of fungi is different for different fungi. Many fungi reproduce both sexually or asexually, while
some reproduce only sexually and some only asexually. Asexual reproduction takes only one parent and sexual
reproduction takes two parents.
Asexual Reproduction
Fungi reproduce asexually through three methods:
a. Spores: Spores are formed by the fungi and released to create new fungi. Have you ever seen a puffball? A
puffball is a kind of fungus that has thousands of spores in a giant ball. Eventually the puffball bursts and
releases the spores in a huge “puff.”
b. Budding: The fungus grows part of its body, which eventually breaks off. The broken-off piece becomes a
“new” organism.
c. Mycelial fragmentation: In this method, a piece of the mycelium splits off of the fungi. A fragmented piece
of the mycelium can eventually produce a new colony of fungi.
Asexual reproduction is faster and produces more fungi than sexual reproduction. Some species of fungi can only
perform asexual reproduction. This form of reproduction is controlled by many different factors, including environmental conditions such as the amount of sunlight and carbon dioxide the fungus receives, as well as the availability
of food.
Sexual Reproduction
Almost all fungi can reproduce through the process of meiosis. Meiosis is a type of cell division where haploid
cells are produced (discussed in chapter titled Cell Division, Reproduction and DNA). But meiosis in fungi is really
different from sexual reproduction in plants or animals.
In plants and animals, meiosis occurs in diploid cells and is a process that produces haploid cells. Remember, a
diploid cell is a cell with two sets of chromosomes, one from each parent. A haploid cell has one set of chromosomes.
In meiosis, four haploid cells are produced. Each haploid cell has half the chromosome number of the parent cell.
However, in fungi, meiosis occurs right after two haploid cells fuse, producing four haploid cells. Mitosis then
produces a haploid multicellular "adult" organism or haploid unicellular organisms. Mitosis is cell division that
creates two genetically identical offspring cells (Figure 9.12 ).
Classification of Fungi
Scientists used to think that fungi were members of the plant kingdom. They thought this because fungi had several
similarities to plants. For example, fungi and plants usually have a leaf or flower that is attached to a stem. Also:
• Fungi and plants have similar structures.
• Plants and fungi live in the same kinds of habitats, such as growing in soil.
• Plants and fungi both have a cell wall, which animals do not have.
9.2. FUNGI
A diagram of how asexual and sexual reproduction work in fungi.
Structure of Fungi
There are a number of characteristics that make fungi different from other eukaryotic organisms:
a. Fungi cannot make their own food like plants can, since they do not have chloroplasts and cannot carry out
photosynthesis. Fungi are more like animals because they have to obtain their food from outside sources.
b. The cell walls in many species of fungi contain chitin. Chitin is a nitrogen-containing material found in the
shells of animals such as beetles and lobsters. The cell wall of a plant is made of cellulose, not chitin.
c. Unlike many plants, most fungi do not have structures that transfer water and nutrients.
d. One characteristic that is unique to fungi is the presence of hyphae, which combine in groups called mycelia,
as described above.
The Evolution of Fungi
Fungi appeared during the Paleozoic Era, a geologic time period lasting from about 570 million to 248 million years
ago. This is also the time when fish, insects, amphibians, reptiles, and land plants appeared. The first fungi most
likely lived in water and had flagella that released spores. The first land fungi probably appeared in the Silurian
period (443 million years ago to about 416 million years ago), the same time period that land plants also first
appeared. See different types of fungi here:
Roles of Fungi
Fungi are found all over the globe in many different kinds of habitats. Fungi even thrive in deserts. Most fungi
are found on land rather than in the ocean, though some species live only in ocean habitats. Fungi are extremely
important to these ecosystems because they are one of the major decomposers of organic material. Scientists have
estimated that there are nearly 1.5 million species of fungi.
Importance of Fungi for Human Use
Humans use fungi for food preparation or preservation and other purposes.
Yeasts are help ferment beer, wine and bread (Figure ?? ).
Some fungi are used in the production of soy sauce and tempeh, a source of protein used in Southeast Asia.
Mushrooms are used in the diet of people all over the globe.
Fungi can produce antibiotics, such as penicillin.
The chitin in the cell walls of fungi has been said to have healing properties.
Edible and Poisonous Fungi
Some of the best known types of fungi are mushrooms, which can be edible or poisonous (Figure 9.13 ).
Many species are grown commercially, but others are harvested from the wild. When you order a pizza with mushrooms or add them to your salad, you are most likely eating Agaricus bisporus, the most commonly eaten species.
Other mushroom species are gathered from the wild for people to eat or for commercial sale. Many mushroom
species are poisonous to humans. Some mushrooms will simply give you a stomach ache, while others may kill you.
Some mushrooms you can eat when they are cooked but are poisonous when raw.
Have you ever eaten blue cheese? Do you know what makes it blue? You guessed it. Fungus. For certain types of
cheeses, producers add fungus spores to milk curds to promote the growth of mold, which makes the cheese blue.
Molds used in cheese production are safe for humans to eat.
Some of the best known types of fungi
are the edible and the poisonous mushrooms.
9.2. FUNGI
Fungi Control of Pests
Some fungi work as natural pesticides. For example, some fungi may be used to limit or kill harmful organisms like
mites, pest insects, certain weeds, worms, and other fungi that harm or kill crops.
Lesson Summary
• Fungi are classified in their own kingdom based on their structures, ways of obtaining food, and on they
• Fungi live with other organisms in symbiotic relationships.
• Fungi reproduce asexually and sexually.
• Fungi appeared during the Paleozoic Era.
• Fungi are widely used in foods, industry, and medicine.
Review Questions
1. What are two characteristics distinguishes fungi from plants?
2. How many species of fungi exist?
3. Name two human diseases caused by fungus.
4. What is a lichen?
Apply Concepts
5. Explain how mycorrhizal symbiosis works.
6. Describe the relationship between the ambrosia beetle and fungi.
7. If you see mold on your bread, what part of the fungus are you observing?
8. Describe the three methods of asexual reproduction in fungi.
Critical Thinking
9. "Sexual reproduction in fungi is similar to sexual reproduction in animals." Explain why this statement is true or
10. You go back in time to talk with scientists who believe that fungi are a type of plant. What will you tell them?
Further Reading / Supplemental Links
• Money, Nicholas, The Triumph of Fungi: A Rotten History. Oxford University Press, 2006.
• Webster, Robert and Weber, Roland, Introduction to Fungi. Cambridge University Press, 2007.
• Moore-Landecker, Elizabeth, Fundamentals of Fungi. Benjamin Cummings, 1996.
Points to Consider
• Plants are fascinating, diverse organisms. Although scientists used to think that fungi were plants, we now
know that plants and fungi are separate. In this lesson we have discussed fungi. Next we start discussing
plants. What do you think sets plants apart from fungi?
9.2. FUNGI
MS Plants
How many questions can you ask about flowers? Why are they so brightly colored? Why are there so many different
kinds? Do all plants have flowers? What are plants with flowers called? Why did plants evolve to have flowers?
What other structure is included in the above image on the plant? Those are berries, or the fruits of the plant. What
is a fruit? Why is the fruit important for a plant? Why did plants evolve to have fruit?
Scientists and naturalists have been asking these questions for centuries. While reading, see if you can answer some
of the above questions and ask some more of your own.
Introduction to Plants
Lesson Objectives
Describe the major characteristics that of organisms in Kingdom Plantae.
Describe plants’ major adaptations for life on land.
Explain plants’ reproductive cycle.
Explain how plants are classified.
Check Your Understanding
• What are the major differences between a plant cell and an animal cell?
• What is photosynthesis?
alternation of generations A lifecycle that alternates between a haploid gametophyte and a diploid sporophyte;
characteristic of plants.
angiosperms Plants with vascular tissue, seeds, and flowers.
cuticle Waxy layer that aids water retention in plants.
gymnosperms Seed plant where seeds are not enclosed by a fruit.
nonvascular plants Plants that do not have vascular tissue to conduct food and water.
phloem Vascular tissue that carries the sugars made during photosynthesis (in the leaves) to other parts of the
seedless vascular plants Plants with vascular tissue but no seeds.
vascular tissue Tissues that conduct food, water, and nutrients in plants.
xylem Vascular tissue responsible for the transport of water and nutrients from the roots to the rest of the plant.
What Are Plants?
Plants have adapted to a variety of environments, from the desert to the tropical rain forest to our lakes and oceans.
In each environment, plants have become crucial to supporting animal life. From tiny mosses to extremely large
trees (Figure 10.1 ), the organisms in this kingdom, Kingdom Plantae, have three main features. They are all:
a. Eukaryotic.
b. Photosynthetic.
c. Multicellular.
Recall that eukaryotic organisms also include animals, protists, and fungi. Eukaryotes have cells with nuclei that
contain DNA and membrane-bound organelles, such as mitochondria. As discussed in the Cell Functions chapter,
photosynthesis is the process by which plants capture the energy of sunlight and use carbon dioxide from the air to
make their own food. Lastly, plants must be multicellular. Recall that some protists are eukaryotic and photosynthetic, but are not considered plants because they are mostly unicellular.
There is great diversity in the plant kingdom from tiny mosses to huge trees.
Adaptations For Life On Land
Before plants evolved, most photosynthetic organisms lived in the water. So, where did plants come from? Evidence
shows that plants evolved from freshwater green algae (Figure 10.2 ). The similarities between green algae and
plants is one piece of evidence. They both have cellulose in their cell walls, and they share many of the same
chemicals that give them color. (For a review of plant cells, see the Cells and Their Structures chapter.) So what
separates green algae, which are protists, from green plants?
The ancestor of plants is green algae.
This picture shows a close up of algae
on the beach.
There are four main ways that plants adapted to life on land, and as a result became different from algae:
a. In plants, the embryo develops inside of the female plant after fertilization. Algae do not keep the embryo
inside of themselves, but release it into water. This was the first feature to evolve that separated plants from
green algae. This is also the only adaptation shared by all plants.
b. Over time, plants had to evolve from living in water to living on land. In early plants, a waxy layer called a
cuticle evolved to help seal water in the plant and prevent water loss. However, the cuticle also prevents gases
from entering and leaving the plant easily. Recall that the exchange of gasses - taking in carbon dioxide and
releasing oxygen - occurs during photosynthesis.
c. To allow the plant to retain water and exchange gases, small pores (holes) in the leaves called stomata also
evolved (Figure 10.3 ). The stomata can open and close depending on weather conditions. When it’s hot and
dry, the stomata close to keep water inside of the plant. When the weather cools down, the stomata can open
again to let carbon dioxide in and oxygen out.
d. A later adaption for life on land was the evolution of vascular tissue. Vascular tissue is specialized tissue that
transports water, nutrients, and food in plants. In algae, vascular tissue is not necessary since the entire body
is in contact with the water and the water simply enters the algae. But on land, water may only be found deep
in the ground. Vascular tissues take water and nutrients from the ground up into the plant, while also taking
food down from the leaves into the rest of the plant. The two vascular tissues are:
a. Xylem: Responsible for the transport of water and nutrients from the roots to the rest of the plant.
Generally (not always), xylem carries water from the roots up to the rest of the plant.
b. Phloem: Carries the sugars made during photosynthesis (in the leaves) to the parts of the plant where
they are needed. Generally (not always), phloem carries sugars down to the rest of the plant.
Stomata are pores in leaves that allow
gasses to pass through but they can be
closed to conserve water.
Plant Reproduction and Life Cycle
The life cycle of a plant is very different from the life cycle of an animal. A human cannot exist unless it is made
entirely of diploid cells (cells with two sets of chromosomes). Plants can live, however, when they are made up of
diploid cells or haploid cells (cells with one set of chromosomes).
Plants alternate between diploid-cell plants and haploid-cell plants. This is called alternation of generations because the plant type alternates from generation to generation. In alternation of generations, the plant alternates
between a sporophyte that has diploid cells and a gametophyte that has haploid cells.
Briefly, alternation of generations can be summarized in the following four steps: follow along in Figure 10.4 as
you read through the steps.
The gametophyte produces the gametes, or sperm and egg, by mitosis. Remember, gametes are haploid.
Then the sperm fertilizes the egg, producing a diploid zygote that develops into the sporophyte.
The diploid sporophyte produces haploid spores by meiosis.
The haploid spores go through mitosis, developing into the gametophyte.
As we will see in the following lessons, the generation in which the plant spends most of its life cycle is different
between various plants. In the plants that first evolved, the gametophyte takes up the majority of the life cycle of the
plant. During the course of evolution, the sporophyte became the major stage of the life cycle of the plant.
In flowering plants, the female gametophyte, the ovary, is found within the sporophyte. The male gametophyte is
the pollen.
In ferns the sporophyte is dominant
and produces spores that germinate
into a gametophyte. After fertilization the
sporophyte is produced. Ferns will be
discussed in further detail in the next
Classification of Plants
Plants are formally divided into 12 phyla (plural for phylum), and these phyla are gathered into four groups. These
four groups are based on the evolutionary history of significant features in plants:
a. Nonvascular plants evolved first, and keep the embryo inside of the reproductive structure after fertilization.
These plants do not have vascular tissue (xylem and phloem).
b. Seedless vascular plants evolved to have vascular tissue after the nonvascular plants, but do not have seeds.
c. Non-flowering plants, or gymnosperms, evolved to have seeds, but do not have flowers.
d. Flowering plants, or angiosperms, evolved to have vascular tissue, seeds, and flowers.
These four groups are the focus of the next two lessons. Figure ?? shows some of the rich diversity of the plant
Lesson Summary
• Plants are multicellular photosynthetic eukaryotes that evolved from green algae.
• Plants have several adaptive features for living on land, including a cuticle, stomata, and vascular tissue.
• Plants are informally divided into four groups: the nonvascular plants, the seedless vascular plants, the nonflowering plants (gymnosperms) and the flowering plants (angiosperms).
Review Questions
1. What is the purpose of the stomata?
2. What term describes the plant life cycle?
3. What is the diploid stage of the alternation of generations?
4. What is the term for plants that lack vascular tissue?
5. What is the term for plants that have flowers and bear fruit?
Apply Concepts
6. Plants evolved from green algae. How are they different from green algae?
7. Describe how plants evolved from water-living organisms to organisms that live on land.
Critical Thinking
8. A scientist mistakes a photosynthetic protist from a plant. However, the protist is not a plant. Explain why.
9. Why do you think plants are necessary for animal life?
Further Reading / Supplemental Links
Points to Consider
Next we discuss seedless plants.
• Can you think of examples of plants that do not have seeds?
• If a plant does not have seeds, how can it reproduce?
Seedless Plants
Lesson Objectives
Name examples of nonvascular seedless plants.
Name examples of vascular seedless plants.
Explain the reproduction strategies of seedless plants.
Describe the ways seedless plants impact humankind.
Check Your Understanding
• What is a plant?
• How are plants classified?
club mosses Seedless vascular plants that resemble mosses.
ferns Seedless vascular plants that have large, divided fronds.
hornworts Seedless nonvascular plants with hornlike sporophytes.
horsetails Seedless vascular plants with hollow, rigid stems.
liverworts Seedless nonvascular plants that can have flattened bodies resembling a liver.
mosses Seedless nonvascular plants with tiny stem-like and stem-like structures.
sporangium Capsule, formed by the sporophyte, which releases spores.
whisk ferns Seedless nonvascular plants with tiny stem-like and stem-like structures.
Seedless Plants
What do you think a forest looked like millions of years ago? Or tens of millions of years ago? Or hundreds of
millions of years ago? Probably very different than today.
Nonvascular seedless plants and vascular seedless plants have had a great impact on all our lives. More than 300
million years ago, during the Carboniferous period, forests looked very different than they do today. Seedless plants
grew as tall as today’s trees in large swampy forests (Figure 10.5 ). The remains of these forests formed the coal
that we depend on today. Although most of these giant seedless plants are now extinct, smaller relatives still remain.
Seedless plants were dominant during
the Carboniferous period as illustrated
by this drawing.
Nonvascular Seedless Plants
Nonvascular seedless plants, as their name implies, lack vascular tissue. Of course, they don’t have seeds either.
As they lack vascular tissue, they also do not have true roots, stems or leaves. Nonvascular plants do often have a
“leafy” appearance though, and can have stem-like and root-like structures. These plants are very short because they
cannot move nutrients and water up a stem.
Nonvascular plants are classified into three phyla:
a. Mosses
b. Hornworts
c. Liverworts
Mosses have a scientific name, Bryophyta. They are most often recognized as the green “fuzz” on damp rocks and
trees in a forest. If you look closely, you will see that most mosses have tiny stem-like and leaf-like structures. This
is the gametophyte stage. Remember that a gametophyte is haploid. The gametophyte produces the gametes that,
after fertilization, develop into the diploid sporophyte. The sporophyte forms a capsule, called the sporangium,
which releases spores (Figure 10.6 ).
Sporophytes sprout up on stalks from
this bed of moss gametophytes. Notice
that both the sporophytes and gametophytes exist at the same time.
Hornworts are part of the phylum Anthocerophyta. The "horn" part of the name comes from their hornlike sporophytes, and “wort” comes from the Anglo-Saxon word for herb. The hornlike sporophytes grow from a base of
flattened lobes, which are the gametophytes (Figure 10.7 ). They usually grow in moist and humid areas.
Liverworts are in the phylum Hepatophyta. They have two distinct appearances: they can either be leafy like
mosses, or flattened and ribbon-like. Liverworts get their name from the type with the flattened bodies, which can
resemble a liver (Figure 10.8 ). Liverworts can often be found along stream beds.
In hornworts the &#8220 horns&#8221
are the sporophytes that rise up from the
leaflike gametophyte.
Liverworts with a flattened ribbon-like
body are called thallose liverworts.
Vascular Seedless Plants
For these plants, the name says it all. Vascular seedless plants have vascular tissue but do not have seeds. Remember
that vascular tissue is specialized tissue that transports water and nutrients throughout the plant. The development of
vascular tissue allowed these plants to grow much taller than nonvascular plants, forming the ancient swamp forests
mentioned previously. Most of these large vascular seedless plants are now extinct, but their smaller relatives still
Seedless vascular plants include:
Club mosses.
Whisk ferns.
Clubmosses, in the phylum Lycophyta, are so named because they can look similar to mosses (Figure 10.9 ).
Clubmosses are not true mosses, though, because they have vascular tissue. The “club” part of the name comes from
club-like clusters of sporangia found on the plants. One type of clubmoss is called the "resurrection plant" because
it shrivels and turns brown when it dries out, but then quickly turns green when watered again.
Clubmosses can resemble mosses but
clubmosses have vascular tissue while
mosses do not.
Ferns, in the phylum Pterophyta, are the most common seedless vascular plants (Figure 10.10 ). They usually have
large divided leaves called fronds. In most ferns, fronds develop from a curled-up formation called a fiddlehead
(Figure 10.11 ). The fiddlehead looks like the curled decoration on the end of a stringed instrument, such as a fiddle.
Leaves unroll as the fiddleheads grow and expand. Ferns grow in a variety of habitats, ranging in size from tiny
aquatic species to giant tropical plants.
FIGURE 10.10
Ferns are common in the understory of
the tropical rainforest.
FIGURE 10.11
The first leaves of most ferns appear
curled up into fiddleheads.
Horsetails, in the phylum Sphenophyta, have hollow, ribbed stems and are often found in marshes (Figure 10.12
). Whorls of tiny leaves around the stem make the plant look like a horse’s tail, but these soon fall off and leave
a hollow stem that can perform photosynthesis (mostly, photosynthesis occurs in leaves). The stems are rigid and
rough to the touch because they are coated with a scratchy mineral. Because of their scratchy texture, these plants
were once used as scouring pads for cleaning dishes.
FIGURE 10.12
Horsetails are common in marshes.
Whisk Ferns
Whisk ferns, in the phylum Psilophyta, have green branching stems with no leaves, so they resemble a whisk broom
(Figure 10.13 ). Another striking feature of the whisk ferns is its spherical yellow sporangia.
Reproduction of Seedless Plants
Seedless plants can reproduce asexually or sexually. Some seedless plants, like hornworts and liverworts, can reproduce asexually through fragmentation. When a small fragment of the plant is broken off, it can form a new
Reproduction in Nonvascular Seedless Plants
Like all plants, nonvascular plants have an alternation of generations life cycle. In the life cycle of the nonvascular
seedless plants, the gametophyte stage is the longest part of the cycle. The gametophyte is photosynthetic.
FIGURE 10.13
Whisk ferns have no leaves and bear
yellow sporangia.
The life cycle of nonvascular seedless plants can be described as follows:
a. The male gametophyte produces flagellated sperm that must swim to the egg formed by the female gametophyte. For this reason, sexual reproduction must happen in the presence of water. Therefore, nonvascular
plants tend to live in moist environments.
b. Following fertilization, the sporophyte forms. The sporophyte is connected to and dependent on the gametophyte.
c. The sporophyte produces spores that will develop into gametophytes and start the cycle over again.
Reproduction in Seedless Vascular Plants
For the seedless vascular plants, the sporophyte stage is the longest part of the cycle, but the cycle is similar to
nonvascular plants. For example, in ferns, the gametophyte is a tiny heart-shaped structure, while the leafy plant we
recognize as a fern is the sporophyte (see Figure 10.13 ).
The sporangia of ferns are often on the underside of the fronds (Figure 10.14 ). Like nonvascular plants, ferns
also have flagellated sperm that must swim to the egg. Unlike nonvascular plants, once fertilization takes place, the
gametophyte will die and the sporophyte will live independently.
Why Seedless Plants Are Important
Seedless Plants Became Coal
The greatest influence of seedless plants have had on human society is in the formation of coal millions of years ago.
When the seedless plants died, became buried deep in the earth, and were exposed to heat and pressure, coal formed.
Now coal is burned to provide energy, such as electricity.
FIGURE 10.14
This fern is producing spores underneath its fronds.
Current Uses
But some seedless plants still have uses in society today. Peat moss, is commonly used by gardeners to improve soils
since it is really good at absorbing and holding water (Figure 10.15 ).
Ferns are also found in many gardens as ornaments. The fiddleheads of certain species of ferns are used in gourmet
food. Some species of ferns, like the maidenhair fern, are used as medicines.
FIGURE 10.15
Sphagnum or peat moss is commonly
added to soil to help absorb water and
keep it in the soil.
Lesson Summary
• Nonvascular seedless plants include mosses, liverworts, and hornworts.
• Vascular seedless plants include clubmosses, ferns, whisk ferns, and horsetails.
• Nonvascular seedless plants spend most of their life cycle in the gametophyte stage, while vascular seedless
plants spend most of their life as a sporophyte.
• The death of seedless plants millions of years ago produced coal.
• Mosses and ferns are used commonly in gardening.
Review Questions
1. What is vascular tissue?
2. What is an example of a nonvascular seedless plant?
3. What is an example of a vascular seedless plant?
4. Compare and contrast the fern gametophyte and sporophyte.
5. What are some of the distinguishing features of horsetails?
6. What does the sporophyte of the hornwort look like?
Apply Concepts
7. Your friend finds a whisk fern and insists it is the same as a fern. Explain why it is different.
8. Explain why the following quote is true: "Clubmoss is not a type of moss."
Critical Thinking
9. "After they died, many seedless plants have been a great benefit to humans." Explain why you agree or disagree
with the statement.
10. Explain to a group of gardeners how they may use seedless plants.
Further Reading / Supplemental Links
Points to Consider
Next we discuss plants with seeds.
• Can you think of examples of plants that have seeds?
• Can you think of a plant that has seeds but no flowers or fruits?
• Why do you think having flowers is beneficial to a plant?
Seed Plants
Lesson Objectives
Describe the importance of the seed.
Explain the ways in which seeds are dispersed.
Define and give examples of gymnosperms.
Define and give examples of angiosperms.
Explain some uses of seed plants.
Check Your Understanding
• What are the two types of seedless plants?
• How do seedless plants reproduce?
anther The pollen-containing structure in a flower.
calyx The sepals collectively; outermost layer of the flower.
carpel Female portion of the flower; consists of stigma, style, and ovary.
complete flowers Flowers that contain all four structures: sepals, petals, stamens, and one or more carpels.
conifers Group of gymnosperms that bear cones; includes spruces, pine, and fir trees.
corolla The petals of a flower collectively are known as the corolla.
dormant Halting growth and development temporarily.
ginkgo Tree known as the living fossil because it is the only species left in the phylum Ginkgophyta.
incomplete flowers Flowers that are missing one or more structures: sepals, petals, stamens, or carpels.
ovary Enlarged part of the carpel where the ovules are contained.
sepals Outermost layer of the flower that is usually leaf-like and green.
stamen The part of the flower consisting of a filament and an anther that produces pollen.
stigma The knoblike section of the carpel where the pollen must land for fertilization to occur.
Seeds and Seed Dispersal
What is a Seed?
If you’ve ever seen a plant grow from a tiny seed, then you might realize that seeds are amazing structures. The seed
allows a plant embryo to survive droughts, harsh winters, and other conditions that would kill an adult plant. The
tiny plant embryo can simply stay dormant, in a resting state, and wait for the perfect environment to begin to grow.
In fact, some seeds can stay dormant for hundreds of years!
Another impressive feature of the seed is that it stores food for the young plant after it sprouts. This greatly increases
the chances that the tiny plant will survive. So being able to produce a seed is a very beneficial adaptation, and as a
result, seed plants have been very successful. Although the seedless plants were here on Earth first, today there are
many more seed plants than seedless plants.
How are Seed Plants Successful?
For a seed plant species to be successful, the seeds must be dispersed, or scattered around in various directions. If
the seeds are spread out in many different areas, there is a better chance that some of the seeds will find the right
conditions to grow. But how do seeds travel to places they have never been before? To aid with seed dispersal, some
plants have evolved special features that help their seeds travel over long distances.
One such strategy is to allow the wind to carry the seeds. With special adaptations in the seeds, the seeds can be
carried long distances by the wind. For example, you might have noticed how the "fluff" of a dandelion moves in
the wind. Each piece of fluff carries a seed to a new location. If you look under the scales of pine cone, you will
see tiny seeds with "wings" that allow these seeds to be carried away by the wind. Maple trees also have specialized
fruits with wing-like parts that help seed dispersal, as shown in Figure 10.16 .
FIGURE 10.16
&#8220 wings&#8221
the wind disperse the seeds.
Some flowering plants grow fleshy fruit that helps disperse their seeds. When animals eat the fruit, the seeds pass
through an animal’s digestive tract unharmed. The seeds germinate after they are passed out with the animal’s feces.
Berries, citrus fruits, cherries, apples, and a variety of other types of fruits are all adapted to be attractive to animals,
so the animals will eat them and spread the seed (Figure 10.17 ).
FIGURE 10.17
Fleshy fruits aid in seed dispersal since
animals eat the fruits and carry the
seeds to a new location.
Some non-fleshy fruits are specially adapted for animals to carry them on their fur. You might have returned from
a walk in the woods to find burrs stuck to your socks. These burrs are actually specialized fruits designed to carry
seeds to a new location.
Plants with "naked" seeds, meaning they are not enclosed by a fruit, are called gymnosperms. Instead, the seeds of
gymnosperms are usually found in cones.
There are four phyla of gymnosperms:
Conifers, members of the phylum Coniferophyta, are probably the gymnosperms that are most familiar to you.
Conifers include pines, firs, spruces, cedars, and the coastal redwood trees in California that are the tallest living
vascular plants.
Conifers have their reproductive structures in cones, but they are not the only plants to have that trait (Figure 10.18
). Conifer pollen cones are usually very small, while the seed cones are larger. Pollen contains gametophytes that
produce the male gamete of seed plants. The pollen, which is a powder-like material, is carried by the wind to
fertilize the seed cones that contain the female gamete (Figure 10.19 ).
FIGURE 10.18
A red pine which bears seeds in cones
is an example of a conifer.
FIGURE 10.19
The end of a pine tree branch bears the
male cones that produce the pollen.
Conifers have many uses. They are important sources of lumber and are also used to make paper. Resins, the sticky
substance you might see oozing out of a wound on a pine tree, are collected from conifers to make a variety of
products, such as the solvent turpentine and the rosin used by musicians and baseball players. The sticky rosin
improves the pitcher’s hold on the ball or increases the friction between the bow and the strings to help create music
from a violin or other stringed instrument.
Cycads, in the phylum Cycadophyta, are also gymnosperms. They have large, finely-divided leaves and grow as
short shrubs and trees in tropical regions. Like conifers, they produce cones, but the seed cones and pollen cones are
always on separate plants (Figure 10.20 ). One type of cycad, the Sago Palm, is a popular landscape plant. During
the Age of the Dinosaurs (about 65 to 200 million years ago), cycads were the dominant plants. So you can imagine
dinosaurs grazing on cycad seeds and roaming through cycad forests.
FIGURE 10.20
Cycads bear their pollen and seeds in
cones on separate plants.
Ginkgoes, in the phylum Ginkgophyta, are unique because they are the only species left in the phylum. Many other
species in the fossil record have gone extinct (Figure 10.21 ). The ginkgo tree is sometimes called a "living fossil,"
because it is the last species from its phylum.
One reason the ginkgo tree may have survived is because it was often grown around Buddhist temples, especially in
China. The ginkgo tree is also a popular landscape tree today in American cities because it can live in polluted areas
better than most plants.
Ginkgoes, like cycads, has separate female and male plants. The male trees are usually preferred for landscaping
because the seeds produced by the female plants smell terrible when they ripen.
Gnetophytes, in the phylum Gnetophyta, are a very small and unusual group of plants. Ephedra is an important
member of this group, since this desert shrub produces the ephedrine used to treat asthma and other conditions.
Welwitschia produces extremely long leaves and is found in the deserts of southwestern Africa (Figure 10.22 ).
Overall, there are about 70 different species in this diverse phylum.
FIGURE 10.21
Ginkgo trees are gymnosperms with
broad leaves.
FIGURE 10.22
One type of gnetophyte is Welwitschia.
Angiosperms, in the phylum Anthophyta, are the most successful phylum of plants. This category also contains the
largest number of individual plants (see Figure 10.23 ). Angiosperms evolved the structure of the flower, so they are
also called the flowering plants. Angiosperms live in a variety of different environments. A water lily, an oak tree,
and a barrel cactus, although different, are all angiosperms.
FIGURE 10.23
Angiosperms are the flowering plants.
The Parts of a Flower
Even though flowers may look very different from each other, they do have some structures in common. Follow
along in Figure 10.24 as the structures are explained below:
• The green outside of a flower that often looks like a leaf is called the sepal (Figure 10.25 ). All of the sepals
together are called the calyx, which is usually green and protects the flower before it opens.
• All of the petals (Figure 10.25 ) together are called the corolla. They are bright and colorful to attract a
particular pollinator, an animal that carries pollen from one flower to another.
• The next structure is the stamen, consisting of the stalk-like filament that holds up the anther, or pollen sac.
The pollen is the male gametophyte.
• At the very center is the carpel, which is divided into three different parts: (1) the sticky stigma, where the
pollen lands, (2) the tube of the style, and (3) the large bottom part, known as the ovary.
The ovary holds the ovules, the female gametophytes. When the ovules are fertilized, the ovule becomes the seed
and the ovary becomes the fruit.
When flowers have all of these parts, they are known as complete flowers. Other flowers may be missing one or
more of these parts and are known as incomplete flowers. Table 10.1 summarizes the parts of the flower.
FIGURE 10.24
A complete flower has sepals petals
stamens and one or more carpels.
FIGURE 10.25
This image shows the difference between a petal and a sepal.
TABLE 10.1: Review the parts of the flower
Flower part
The green outside of the flower.
All of the sepals together, or the outside of the flower.
The petals of a flower collectively.
The part of the flower anther that produces pollen.
Stalk that holds up the anther.
The structure that contains pollen in a flower.
“Female” part of the flower; includes the stigma, style,
and ovary.
The part of the carpel where the pollen must land for
fertilization to occur.
Tube that makes up part of the carpel.
Larged bottom part of the carpel where the ovules are
How Do Angiosperms Reproduce?
Flowering plants can reproduce two different ways:
a. Self-pollination: Pollen falls on the stigma of the same flower. This way, a seed will be produced that can turn
into a genetically identical plant.
b. Cross-fertilization: Pollen from one flower travels to a stigma of a flower on another plant. Pollen travels from
flower to flower by wind or by animals. Flowers that are pollinated by animals such as birds, butterflies, or
bees are often colorful and provide nectar, a sugary reward, for their animal pollinators.
Why Are Angiosperms Important to Humans?
Angiosperms are important to humans in many ways, but the most significant role of angiosperms is as food. Wheat,
rye, corn, and other grains are all harvested from flowering plants. Starchy foods, such as potatoes, and legumes,
such as beans, are also angiosperms. And as mentioned previously, fruits are a product of angiosperms to increase
seed dispersal and are also nutritious foods.
There are also many non-food uses of angiosperms that are important to society. For example, cotton and other
plants are used to make cloth, and hardwood trees are used for lumber.
Lesson Summary
Seeds consist of a dormant plant embryo and stored food.
Seeds can be dispersed by wind or by animals that eat fleshy fruits.
Gymnosperms, seed plants without flowers, include conifers, cycads, gingkoes, and gnetophytes.
Angiosperms are flowering plants.
Seed plants provide many foods and products for humans.
Review Questions
1. How do seeds help plants adapt to their environment?
2. What are two ways that plants disperse their seeds?
3. What are some examples of gymnosperms?
4. Firs, spruces, and pines belong to what group of gymnosperms?
5. Where is the pollen stored in a flower?
6. How are plants pollinated?
Apply Concepts
7. What is the purpose of a plant developing a fruit?
8. How are gymnosperms and angiosperms different?
9. What are some uses that seed plants have for humans?
10. Why is the ginkgo tree considered a “living fossil”?
Think Critically
11. Why did angiosperms evolve the ability to produce flowers? Use the terms "adaptation" and "environment" in
your explanation.
Further Reading / Supplemental Links
• frances.cardillo/plants/intro/plantmen.html
Points to Consider
Now that we have discussed the types of plants, we turn to plant responses.
• Do you think plants can respond to their environment? Why or why not?
• How might plants and fruit change colors?
• How do you think trees know when it’s time to lose their leaves?
Plant Responses
Lesson Objectives
• List the major types of plant hormones and the main functions of each.
• Define tropism and explain examples of tropisms.
• Explain how plants sense the changes of seasons.
Check Your Understanding
• Why do plants need sunlight?
abscisic acid Plant hormone involved in maintaining dormancy and closing the stomata.
apical dominance Suppressing the growth of the side branches of a plant.
auxin Plant hormone involved in tropisms and apical dominance.
cytokinins Plant hormone involved in cell division.
ethylene Plant hormone involved in fruit ripening and abscission.
gibberellins Plant hormone involved in seed germination and stem elongation.
gravitropism Plant growth towards or away from the pull of gravity.
hormones Chemical messengers that signal responses to stimuli.
phototropism Plant growth towards or away from light.
thigmotropism Differential plant growth in response to contact with an object.
tropism Plant growth response towards or away from a stimulus.
Plant Hormones
Plants may not move, but that does not mean they don’t respond to their environment. Plants can sense gravity,
light, touch, and seasonal changes. For example, you might have noticed how a house plant bends towards a bright
window. Plants can sense and then grow toward the source of light. Scientists say that plants are able to respond to
"stimuli," or something-usually in the environment-that results in a response. For instance, light is the stimulus, and
the plant moving toward the light is the "response."
Hormones are special chemical messengers that help plants respond to stimuli in their environment. In order for
plants to respond to the environment, their cells must be able to communicate with other cells. Hormones send
messages between the cells. Animals, like humans, also have hormones, such as testosterone or estrogen, to carry
messages from cell to cell. Animal hormones will be discussed in the Controlling the Body chapter. In both plants
and animals, hormones travel from cell to cell in response to a stimulus and also activate a specific response.
Types of Plant Hormones
Five different types of plant hormones are involved in the main responses of plants. Their functions are listed in
Table 10.2 .
TABLE 10.2:
Abscisic Acid
Fruit ripening and abscission
Break the dormancy of seeds and buds; promote growth
Promote cell division; prevent senescence
Close the stomata ; maintain dormancy
Involved in tropisms and apical dominance
Ethylene has two functions. It (1) helps ripen fruit and is (2) involved in the process of abscission, the dropping of
leaves, fruits and flowers. When a flower is done blooming or a fruit is ripe and ready to be eaten, ethylene causes
the petals or fruit to fall from a plant (Figure 10.26 and Figure 10.27 ).
Ethylene is an unusual plant hormone because it is a gas. That means it can move through the air, and a ripening
apple can cause another apple to ripen, or even over-ripen. That’s why one rotten apple spoils the whole barrel!
Some farmers spray their green peppers with ethylene gas to cause them to ripen faster-and become red peppers.
You can try to see how ethylene works by putting a ripe apple or banana with another unripe fruit in a closed
container or plastic bag–what do you think will happen to the unripe fruit?
Gibberellins are hormones that cause the plant to grow.
When gibberellins are applied to plants by scientists, the stems grow longer. Some gardeners or horticulture scientists
add gibberellins to increase the growth of plants. Dwarf plants (small plants), on the other hand, have low levels of
gibberellins (Figure 10.28 ). Another function of gibberellins is to stop dormancy (resting) time of seeds and buds.
Gibberellins signal that it’s time for a seed to germinate (grow) or for a bud to open.
FIGURE 10.26
The hormone ethylene is signaling
these tomatoes to ripen.
FIGURE 10.27
The hormone ethylene causes flower
petals to fall from a plant a process
known as abscission.
FIGURE 10.28
Dwarf plants like this bonsai tree often have unusually low concentrations
of gibberellins.
Cytokinins are hormones that cause plant cells to divide. Cytokinins were discovered from attempts to grow plant
tissue in artificial (unnatural) environments (Figure 10.29 ). Cytokinins prevent senescence, or the process of aging.
So florists sometimes apply cytokinins to cut flowers, so they do not get old and die.
FIGURE 10.29
Cytokinins promote cell division and are
necessary for growing plants in tissue
culture. A small piece of a plant is
placed in sterile conditions to regenerate a new plant.
Abscisic Acid
Abscisic Acid is misnamed because it was once believed to play a role in abscission (the dropping of leaves, fruits
and flowers), but we now know abscission is caused by ethylene. The actual role of abscisic acid is to close the
stomata and maintain dormancy (resting). When a plant is stressed due to lack of water, abscisic acid tells the
stomata to close. This prevents water loss through the stomata.
When the environment is not good for a seed to germinate (begin to grow), abscisic acid signals for the dormancy
period of the seed to continue. Abscisic acid also tells the buds of plants to stay in the dormancy stage. When
conditions improve, the levels of abscisic acid drop and the levels of gibberellins increase, signaling that is time to
break dormancy (Figure 10.30 ).
FIGURE 10.30
A decrease in levels of abscisic acid allows these buds to break dormancy and
put out leaves.
Auxins are hormones that play a role in plant growth.
Auxins produced at the tip of the plant are involved in apical dominance, when the main central stem grows more
strongly than other stems and branches. When the tip of the plant is removed, the auxins are no longer present and
the side branches begin to grow. This is why pruning (cutting off branches) helps produce a fuller plant with more
branches. You actually need to cut off branches off a plant for it to grow more branches! Auxins are also involved
in tropisms, which will be discussed in the next section.
Plants may not be able to move, but they are able to change how they grow in response to their environment. Growth
toward or away from a stimulus is known as a tropism (Table 10.3 ). The auxins allow plants to curve its growth
specific directions. The auxin moves to one side of the stem, where it starts a chain of events that cause cell growth
on just that one side of the stem. With one side of the stem growing faster than the other, the plant begins to bend.
TABLE 10.3:
Type of Tropism
You might have noticed that plants bend towards the light. This is an example of a tropism where light is the stimulus,
known as phototropism (Figure 10.31 ). To obtain more light for photosynthesis, leaves and stems grow towards
the light. On the other hand, roots grow away from light. This is beneficial for the roots because they need to obtain
water and nutrients from deep within the ground.
FIGURE 10.31
These seedlings bending toward the
sun are displaying phototropism.
So, how do the roots of seeds know to grow downward? How do the roots know which way is up? Gravitropism
is a growth towards or away from the pull of gravity (Figure 10.32 ). Again, the hormone auxin is involved in this
response. Auxin builds up on the lower side of the stem, encouraging growth on this side of the stem and causing it
to bend upwards over time. Shoots also show a gravitropism, but in the opposite direction. If you place a plant on its
side, the stem and new leaves will curve upwards. Shoots are new plant growth. Shoots can include stems, flowering
stems with flower buds, and leaves.
Plants also have a touch response, called thigmotropism. If you have ever seen a morning glory or the tendrils of
a bean plant twist around a pole, then you know that plants must be able to sense the pole. Thigmotropism works
much like the other tropisms. The plant grows straight until it comes in contact with the pole. Then the side of the
stem in contact with the pole grows slower than the opposite side of the stem. This causes the stem to bend around
the pole.
FIGURE 10.32
These shoots are exhibiting gravitropism because they are growing
against the pull of gravity.
See the following link for an example of thigmotropism:
Seasonal Changes
Have you seen the leaves of plants change colors? What time of year does this happen? What causes it to happen?
Plants can sense changes in the seasons. Leaves change color and drop each autumn in certain climates (Figure
10.33 ).
FIGURE 10.33
Leaves changing color is a response to
the shortened length of the day in autumn.
Certain flowers, like poinsettias, only bloom during the winter. And in the spring, the winter buds on the trees break
open and the leaves start to grow. How do plants detect time of year?
Although you might detect the change of seasons by the change in temperature, this is not the way plants know the
seasons are changing. Plants determine the time of year by the length of the day. Because of the tilt of the Earth,
during winter days, there are less hours of light than during summer days. That’s why during the winter it may start
getting dark very early during the evening and even stay dark while you’re getting ready for school the next morning.
But in the summer it will be bright early in the morning and the sun may not set until late that night. With their
hormones, plants can sense the differences in day length.
For example, in the fall when the days start to get shorter, the trees sense that there is less sunlight. The hormones
are stimulated and they send messages telling the leaves to change colors and fall from the plant.
If a plant kept it leaves over the winter, would it be able to perform photosynthesis? Not very much because it would
be too dark. So, the plant sheds its leaves during the winter to rest and then regrows the leaves during the spring and
summer months to make use of the increase in sunlight.
Lesson Summary
• Plant hormones are chemical signals that control different processes in plants.
• A plant tropism is growth towards or away from a stimulus such as light or gravity.
• Many plants go through seasonal changes after detecting differences in day length.
Review Questions
1. What is the term for dropping fruits, flowers, or
2. What hormone is involved with fruit ripening?
3. What hormone is involved in tropisms?
4. What is phototropism?
Apply Concepts
5. Explain how are hormones involved in seed germination.
6. Cells begin to divide in the stem of the plant. What hormones are involved in this process?
7. The tendril of a bean meets the a metal pole. What will happen to the tendril and why?
8. How do plants detect the change in seasons?
9. Explain two seasonal responses in plants.
Critical Thinking
10. A scientist hypotheses that plants lose their leaves due to a colder temperatures in the winter. Explain why this
hypothesis is untrue.
Further Reading / Supplemental Links
Points to Consider
In the next chapter we will turn our attention to animals.
• List some ways you think animals are different from plants.
• What characteristics do you think define an animal?
• Can you think of examples of animals that do not have hard skeletons?
11.3 W ORMS
MS Introduction to
The above image shows an organism from the phylum Cnidaria. What does that mean? Good question.
You may call the above organism a jellyfish. But a jellyfish is not a fish at all, although it is a part of the animal
kingdom. Kingdoms are divided up into phyla, and this organism is classified in the phylum called Cnidaria.
How is the above organism different from you? What is its body shape? Does it have a brain? Does it have a way
to digest food? Can it eat like us? Can it breathe? These are the types of questions scientists ask when they classify
organisms, or put them into categories.
This chapter will explore how different invertebrates, organisms without a backbone, are classified into different
categories. After reading this chapter, see if you can do a better job asking the right questions to put organisms into
specific categories.
Overview of Animals
Lesson Objectives
• List the characteristics that define the animal kingdom.
• Define and give examples of the invertebrates.
Check Your Understanding
• What are the main differences between an animal cell and a plant cell?
• How do animals get their energy?
bilateral symmetry Body plan in which the left and right side are mirror images.
complete digestive tract A digestive tract with two openings, a mouth and anus.
incomplete digestive tract A digestive tract with only one opening.
invertebrate Animal without a backbone.
radial symmetry A body plan in which any cut through the center results in two identical halves.
segmentation A body plan that has repeated units or segments.
Classification of Animals
How are animals different from other forms of life? Recall that all animals are eukaryotic, meaning that they have
cells with true nuclei and membrane-bound organelles. Animals are also multicellular. Remember that protists
can be unicellular. Because animals are multicellular, animal cells can be organized into tissues, organs, and organ
systems. Finally, animals are heterotrophic, meaning they must eat nutrients (food) that come from outside of their
bodies in order to create energy (Figure 11.1 ).
Animals are heterotrophs meaning they
must eat to get molecules necessary for
their growth and energy.
How Are Animals Classified?
Recall that each kingdom of life, including the animal kingdom, is divided into smaller groups called phyla (singlular
= phylum) based on their shared characteristics. There are 38 animal phyla. For example, the phylum Mollusca
largely consists of animals with shells, like snails and clams. Animals were classfied by how they look, and now
DNA evidence is used to confirm that organisms in a particular phylum are related.
What characteristics are used to classify animals?
1. Body Symmetry: One example of a physical characteristic used to classify animals is body symmetry. In organisms that show radial symmetry, such as sea stars, the body is organized like a circle (Figure 11.2 ). Therefore,
any cut through the center of the animal creates two identical halves.
Other animals, such as humans and worms, show bilateral symmetry, meaning their left and right sides are mirror
2. Animals are also often classified by their body structure. For example, segmentation, the repetition of body parts,
defines one phylum of worms (Figure 11.3 ). Animals that have a true body cavity, defined as a fluid-filled space,
and internal organs (like humans) are also classified in separate phyla from those animals that do not have a true
body cavity.
3. The structure of the digestive system of animals can also be used as a characteristic for classification. Animals
with incomplete digestive tracts have only one opening in their digestive tracts, while animals with complete
digestive tract have two openings, the mouth and anus.
In this chapter we will focus on invertebrates, animals that do not have a backbone.
What Are Invertebrates?
Besides being classified into phyla, animals are also often characterized as being invertebrates or vertebrates. These
are terms based on the skeletons of the animals. Vertebrates have a backbone made of bone or cartilage, while
Sea stars are radially symmetrical.
A segmented body plan defines the phylum that includes the earthworms.
invertebrates have no backbone. All vertebrate organisms are in the phylum Chordata, while invertebrates make up
several diverse phyla, some of which are listed in Table 11.1 .
TABLE 11.1: The invertebrates, animals without a backbone, include a variety of organisms
Jellyfish, corals
Flatworms, tapeworms
Nematodes, heartworm
Snails, clams
Earthworms, leeches
Insects, crabs
Sea stars, sea urchins
Invertebrates include insects, earthworms, jellyfish, sea stars, and a variety of other animals. Figure 11.4 shows an
example of a familiar invertebrate, the snail.
In the next lessons we will discuss some of phyla within the animal kingdom that contain invertebrates.
Snails are an example of invertebrates
animals without a backbone.
Lesson Summary
• Animals are multicellular, eukaryotic heterotrophs.
• Animals can be classified using physical characteristics, such as symmetry.
• Invertebrates are animals without a backbone.
Review Questions
1. What are three main features that define the animal kingdom?
2. What does heterotrophic mean?
3. What defines the invertebrates?
4. What are some examples of invertebrates?
Apply Concepts
5. What is the difference between animals that show radial and bilateral symmetry?
6. Name two examples of animals, not included in this lesson, which show bilateral symmetry.
7. What are some examples of animals that show radial symmetry that are not included in this lesson?
8. Do humans have a body cavity? What kind of organs are found there?
9. What is the difference between an incomplete and complete digestive system?
10. How is segmentation used to classify animals?
Critical Thinking
11. While diving in the ocean, you find a circular squishy animal that does not contain bones. How will you describe
this animal to a scientist? What kind of symmetry does it show? Is it an invertebrate or vertebrate?
Further Reading
Points to Consider
The first invertebrates we will look at are the sponges and cnidarians.
• What do you think that jellyfishes and corals have in common?
• Think of some examples of animals that show bilateral symmetry, meaning that the left side is a mirror image
of the right?
Sponges and Cnidarians
Lesson Objectives
• Describe the key features of sponges.
• Describe the key features of cnidarians.
• List examples of cnidarians.
Check Your Understanding
• How are animals classified?
• What is an invertebrate?
cnidarians Invertebrates that have radial symmetry; includes the jellyfish.
corals Cnidarians that live on ocean reefs in colonies.
gastrovascular cavity A large cavity having both digestive and circulatory functions.
medusa Cnidarian with a bell-shaped body, with the mouth and tentacles facing downward, such as a jellyfish.
nematocysts Specialized cells in cnidarians that can release a small thread-like structure and toxins to capture
polyp Cnidarian with a cup-shaped body directed upward.
sessile Permanently attached and not freely moving.
Ocean Invertebrates
The ocean is home to many different types of organisms, including phytoplankton, zooplankton, fish. Phytoplankton,
tiny photosynthetic organisms that float in the water, make their own food from the energy of the sun. Small water
animals, known as zooplankton, and larger animals, such as fish, use phytoplankton as a source of food. These
animals can then be eaten by larger water animals, such as larger fish and sharks.
Among the different types of animals that live in the ocean, the sponges and cnidarians are important invertebrates.
The Sponges are believed to be one of the most ancient forms of animal life on earth. The cnidarians, which include
the jellyfish, also are among the oldest and most unusual animals on earth.
What do you think of when you think of a sponge? Something to wash the dishes with? Well, you are right. Up
until recently, people took sponges out of the ocean and used them to clean their dishes. Now, we make sponges out
of unnatural materials. But the organisms still live in the ocean.
Sponges are classified in the phylum Porifera, from the Latin words meaning "having pores." These pores allow
the movement of water into the sponges’ sac-like bodies (Figure 11.5 ). Sponges pump water through their bodies
because they are sessile, meaning they cannot move, and filter feeders, meaning they must filter the water to separate
organisms and nutrients they want to eat from those they do not.
Sponges have tube-like bodies with
many pores.
Sponges evolved earlier than other animals, so they do not have brains, stomachs, or other organs. In fact, sponges do
not even have true tissues. Instead, their bodies are made up of specialized cells that do specific jobs. For example,
some cells control the flow of water into and out of the sponge by increasing or decreasing the size of the pores.
Cnidarians, in the phylum Cnidaria, include organisms such as the jellyfish (Figure 11.6 ) and sea anemones (Figure
11.7 and Figure 11.8 ) that are found in shallow ocean water. You might recognize that these animals can give you
a painful sting if you step on them. That’s because cnidarians have stinging cells known as nematocysts. When
touched, the nematocysts release a thread of poison that can be used to paralyze prey-usually food for the jellyfish.
The body plan of cnidarians is unique because these organisms show radial symmetry, meaning that they have a
circular body plan, and any cut through the center of the animal leaves two equal halves.
The cnidarians have two basic body forms:
a. Polyp: The polyp is a cup-shaped body with the mouth facing upward, such as a sea anemone.
b. Medusa: The medusa is a bell-shaped body with the mouth and tentacles facing downward, such as a jellyfish.
Unlike the sponges, the cnidarians are made up of true tissues. The inner tissue of a cnidarian is called the gastrovascular cavity, a large space that helps the organism digest and move nutrients around the body. The cnidarians
also have nerve tissue organized into a net-like structure. Cnidarians do not have true organs, however.
Jellyfish have bell-shaped bodies with
Sea anemones can sting and trap fish
with their tentacles.
Cnidarian Colonies
Some types of cnidarians are also known to form colonies. Two examples are described below.
1. The Portuguese Man o’ War looks like a single organism but is actually a colony of polyps (Figure 11.9 ).
One polyp is filled with air to help the colony float, while several feeding polyps hang below with tentacles, full of
nematocysts. The Portuguese Man o’ War is known to cause extremely painful stings to swimmers and surfers who
accidentally brush up against them in the water.
2. Coral reefs look like big rocks, but they are actually alive. They are built from cnidarians called corals (Figure
One type of sea anemone is home to
the clownfish. The clownfish and the
sea anemone help each other survive.
Clownfish are the only fish that do not
get stung by the tentacles of the sea
anemone. The clownfish while being
provided with food cleans away fish and
algae leftovers from the anemone.
The Portuguese Man o´ War can deliver
debilitating stings with its tentacles.
11.10 ). The corals are sessile polyps that can use their tentacles to feed on ocean creatures that pass by. Their
skeletons are made up of calcium carbonate, which is also known as limestone. Over long periods of time, their
skeletons build on each other to produce large structures known as coral reefs. Coral reefs are important habitats for
many different types of ocean life.
FIGURE 11.10
Corals are colonial cnidarians.
Lesson Summary
• Sponges are sessile filter feeders without true tissues.
• Cnidarians, such as jellyfish, have radial symmetry and true tissues.
• Some cnidarians form colonies, such as the Portuguese Man o’ War and corals.
Review Questions
1. What is the only animal to lack true tissues?
2. In what phylum are the sponges?
3. How do sponges gain nutrition?
4. Where are most cnidarians found?
5. What are some examples of cnidarians?
Apply Concepts
6. Cnidarians show radial symmetry. What does this mean?
7. How do cnidarians sting their prey?
8. Describe the nervous system of the cnidarians.
9. How is a jellyfish different from a Portuguese Man o’ War?
10. How are coral reefs built?
Critical Thinking
11. Which organisms do you think evolved first–the sponge or the cnidarian? Explain your answer with two pieces
of evidence from the lesson.
Further Reading / Supplemental Links
Points to Consider
Next we discuss worms.
• How do you think that worms are different from sponges and cnidarians?
• How do you think that worms might be similar to sponges and cnidarians?
Lesson Objectives
• Describe the major features of the flatworms.
• Describe the major features of the roundworms.
• Describe the major features of the segmented worms.
Check Your Understanding
• In terms of body structure, what does segmentation refer to?
• What is a body cavity?
cephalization Having a head region with a concentration of sensory organs and central nervous system.
hydroskeleton Fluid-filled body cavity that provides support for muscle contraction.
tapeworms Intestinal parasites in the phylum Platyhelminthes.
What are Worms?
The word "worm" is not very scientific. But it is a word that informally describes animals that have long bodies with
no arms or legs. Worms show bilateral symmetry, meaning that the right side of their bodies is a mirror of the left.
Worms live in many different types of environments, including in the ocean, in fresh water, on land, and as parasites
of plants and animals.
Three types of worms with different body types will be explored in this lesson:
a. Flatworms, which have ribbon-like bodies with no body cavity.
b. Roundworms, which have a body cavity but no segments.
c. Segmented worms, which have both a body cavity and segmented bodies.
Worms in the phylum Platyhelminthes are called flatworms because they have flattened bodies. Some species of
flatworms are free-living organisms that feed on small organisms and rotting matter. These types of flatworms
include marine flatworms and fresh-water flatworms such as Dugesia (Figure 11.11 and Figure 11.12 ).
Other types of flatworms are parasitic and live inside another organism, called a host, in order to get the food and
energy they need. For example, tapeworms have a head-like area with tiny hooks that help the worm attach to the
intestines of an animal host (Figure 11.13 and Figure 11.14 ).
FIGURE 11.11
Dugesia is a type of flatworm with a
head region and eyespots.
FIGURE 11.12
Marine flatworms can be brightly colored.
Characteristics of Flatworms
The main characteristics of flatworms can be summarized as follows:
a. Flatworms have no true body cavity and an incomplete digestive system, meaning that the digestive tract has
11.3. WORMS
FIGURE 11.13
Tapeworms are parasitic flatworms that
live in the intestines of their hosts. They
can be very long.
FIGURE 11.14
Tapeworms attach to the intestinal wall
with a head region that has hooks and
only one opening.
b. Flatworms do not have a respiratory system, so they have pores that allow oxygen to enter through their body.
c. There are no blood vessels in the flatworms. Their gastrovascular cavity helps them to digest food and to send
nutrients throughout the body.
d. The flatworms have a ladder-like nervous system with a distinct head region that includes nerve cells and
sensory organs, such as eyespots. The development of a head region, called cephalization, evolved at the
same time as bilateral symmetry in animals.
The phylum Nematoda includes non-segmented worms known as nematodes or roundworms (Figure 11.15 ).
Characteristics of Flatworms
There are specific differences between the flatworms and the roundworms.
a. Unlike the flatworms, the roundworms have a body cavity with internal organs.
b. A roundworm has a complete digestive system, which includes both a mouth and an anus. They also include
a large digestive organ known as the gut.
c. Roundworms also have a simple nervous system with a primitive brain. The nerves are connected from the
top to the bottom of the body.
Roundworms can be free-living organisms, but they are probably best known for their role as significant plant and
animal parasites. Heartworms, which cause serious disease in dogs while living in the heart and blood vessels, are
a type of roundworm. Roundworms can also cause disease in humans. Elephantiasis, a disease characterized by the
extreme swelling of the limbs, is caused by infection with a type of roundworm (Figure 11.16 ).
Segmented Worms
The phylum Annelida includes segmented worms, such as the common earthworm, some marine worms, and leeches
(Figure 11.17 and Figure 11.18 ). These worms are known as the segmented worms because their bodies are
segmented, or separated into repeating units. Most segmented worms feed on dead organic matter, while leeches can
live in freshwater and suck blood from host organisms.
Characteristics of Segmented Worms
a. Segmented worms have a well-developed body cavity filled with fluid, which serves as a hydroskeleton, a
supportive structure that helps move the worm’s muscles.
b. Segmented worms also tend to have organ systems that are more developed than the roundworms or flatworms.
Earthworms, for example, have a complete digestive tract, including an esophagus and intestines. The circulatory system consists of paired hearts and blood vessels, while the nervous system consists of the brain and a
ventral nerve cord.
Table 11.2 compares the three worm phyla.
11.3. WORMS
FIGURE 11.15
Nematodes can be parasites of plants
and animals.
FIGURE 11.16
One roundworm parasite causes elephantiasis a disease characterized by
swelling of the limbs.
FIGURE 11.17
Earthworms are segmented worms.
FIGURE 11.18
11.3. WORMS
TABLE 11.2: Comparison of the three phyla containing worms.
Type of Worm
Body Cavity
Digestive System
Lesson Summary
• The flatworms have no true body cavity and include free-living Dugesia and parasitic tapeworms.
• The roundworms, which can also be parasitic or free-living, are non-segmented worms with a complete digestive tract and a primitive brain.
• The segmented worms include the common earthworm and leeches.
Review Questions
1. What is cephalization?
2. Name a parasitic flatworm.
3. Name a parasitic segmented worm.
4. Earthworms are in what phylum?
Apply Concepts
5. Are all worms classified into a single phylum?
6. Describe the respiratory system of the flatworms.
7. How does the body plan of the roundworms differ from that of the flatworms?
8. Describe the digestive system of roundworms.
9. What features distinguish Phylum Annelida from the other worms?
10. Describe the skeletal system of a segmented worm.
Critical Thinking
11. Which phylum includes worms with organs that are most similar to the organs found in humans? Support your
answer with three pieces of evidence.
Further Reading / Supplemental Links
Points to Consider
Next we further our discussing of the invertebrates.
• Can you think of some invertebrates other than those discussed in this chapter?
• How would these other invertebrates be more advanced compared to worms?
11.3. WORMS
MS Other Invertebrates
What do spiders, clams, and grasshoppers all have in common? They are all invertebrates. So is the above organism,
commonly known as a starfish. As you know, even though all of these organisms are invertebrates, they are very
different from each other. They are in four different phyla.
You may know where a spider lives and you may even know some things about how it catches its prey. But where
does a starfish live? Does it move? How does it reproduce? How does it eat? If a starfish is an invertebrate,
how come it looks so different from other invertebrates? These are all questions that scientists ask when they are
classifying an organism.
Scientists and naturalists now classify starfish in a phylum called Echinodermata, one of four phyla explored in this
Lesson Objectives
• Discuss what characteristics define mollusks.
• Describe the different types of mollusks.
• Explain why mollusks are important.
Check Your Understanding
• What is an invertebrate?
• How are animals classified?
mantle A fold of outer skin lining the shell of mollusks; releases calcium carbonate that is used to create the
external shell.
nacre The iridescent inner shell layer produced by some bivalves, some gastropods, and some cephalopods; also
known as mother of pearl.
pearl The hard, round object produced within the mantle of a living shelled mollusk.
radula A molluscan feeding structure, composed mostly of chitin.
What are Mollusks?
When you take a walk along a beach, what do you find there? Sand, the ocean, lots of sunlight. You may also find
shells. The shells you find are most likely left by organisms in the phylum Mollusca. On the beach, you can find
the shells of many different mollusks, including clams, mussels, scallops, oysters, and snails. Their glossy pearls,
mother of pearl, and abalone shells are like pieces of jewelry (see Figure 12.1 , Figure 12.2 , Figure 12.3 , and
Figure 12.4 ).
The Phylum Mollusca
Mollusks belong to the phylum Mollusca. The mollusk body is often divided into different parts:
a. A head with eyes or tentacles
On the beach you can find a wide variety of mollusks.
Pearls being removed from oysters.
The inside of a bivalve one of the mollusk classes described in &#8220 Types
of Mollusks&#8221
mother of pearl.
Shells of marine mollusks including
b. A muscular foot and a mass housing the organs. In most species, the muscular foot helps the mollusk move.
c. A mantle, or fold of the outer skin lining the shell. The mantle also releases calcium carbonate that creates
the external shell, just like the ones you find on the beach.
d. The majority of ocean mollusks have a gill or gills to absorb oxygen from the water.
e. All species have a complete digestive tract that begins at the mouth and runs to the anus.
f. Many species have a feeding structure, the radula, found only in mollusks. The radula is made mostly of
chitin, a tough, semitransparent substance. Radulae range from structures used to scrape algae off rocks to the
beaks of squid and octopuses.
What Are the Mollusks Related to?
Mollusks are most likely related to organisms in the phylum Annelida, like the earthworm and leech. Scientists
believe they are related because when they are in the early stage of development, they look very similar. Unlike
annelids, however, mollusks do not have body segmentation, and their body shape is usually quite different as well.
How Big Are Mollusks?
The giant squid (Figure 12.5 ), which until recently had not been observed alive in its adult form, is one of the largest
organisms in the phylum Mollusca. However, the colossal squid is even larger and can grow up to 46 feet long. The
smallest mollusks are snails that are microscopic in size.
The colossal squid one of the largest
invertebrates here measures 30 feet in
Types of Mollusks
There are approximately 160,000 living species and probably 70,000 extinct species of mollusks. They are typically
divided into ten classes, of which two are extinct. The living classes are listed in Table 12.1 . Which classes are you
most familiar with?
As you can see, the majority of mollusk species live in ocean environments, and many of them are found in the
shallow waters. Freshwater species are mostly bivalves and gastropods. Some gastropods, like land snails and slugs,
live on land.
TABLE 12.1:
Molluscan Class
Number of Species
Deep ocean
Deep ocean
Rocky marine shorelines
Deep ocean
150,000 (80% of living
molluscan diversity)
Marine (some limpets live
in deep ocean around
hot hydrothermal vents),
freshwater, and terrestrial
Marine (some clams live
in deep ocean around hot
hydrothermal vents) and
Worm-like organisms
Worm-like organisms
Chitons (Figure 12.6 )
Limpet (cone shaped)-like
Abalone, limpets, conch,
nudibranchs (sea slugs),
sea hares (large sea-slug),
sea butterfly, snails, and
slugs (Figure 12.7 , Figure 12.8 , and Figure Figure 12.9 ).
advanced of all invertebrates; include squid,
octopus, cuttlefish, and
nautilus (Figure Figure
12.10 ).
Most bivalves are filter
feeders (matter and food
particles are filtered from
the water, typically by
passing the water over a
specialized filtering structure); bivalves include
clams, oysters, scallops,
and mussels.
Tusk shells
A chiton and sea anemones at a tide
An example of a gastropod species the
ostrich foot.
A picture of limpets
A sea slug underwater
FIGURE 12.10
A Caribbean reef squid an example of a
Importance of Mollusks
Mollusks are important in a variety of ways, including as food, for decoration, in jewelry, and in scientific studies.
They are even used as roadbed material and in vitamin supplements.
Mollusks as Food
Edible species of mollusks include numerous species of clams, mussels, oysters, scallops, marine and land snails,
squid, and octopus. Many species of mollusks, such as oysters, are farmed in order to produce more than could be
found in the wild (Figure 12.11 ).
Mollusks in Decoration and Jewelry
Two natural products of mollusks used for decorations and jewelry are pearls and nacre, or mother of pearl. A pearl
is the hard, round object produced within the mantle of a living shelled mollusk. Fine quality natural pearls have
been highly valued as gemstones and objects of beauty for many centuries. The most desirable pearls are produced
by oysters and river mussels.
Nacre is an iridescent inner shell layer produced by some bivalves, some gastropods, and some cephalopods. It has
been used in sheets on floors, walls, countertops, doors, and ceilings. It is also inserted into furniture; it can be found
in buttons, watch faces, knives, guns, and jewelry; and is used as decorations on various musical instruments.
Mollusks in Scientific Studies
Several mollusks are ideal subjects for scientific investigation, especially in the area of neurobiology. The giant
squid has a sophisticated nervous system and a complex brain for study. The California sea slug, also called the
California sea hare, is used in studies of learning and memory because it has a simple nervous system, consisting
of just a few thousand large, easily identified neurons. These neurons are also responsible for a variety of learning
tasks. Some slug brain studies have even allowed scientists to better understand human brains!
FIGURE 12.11
An oyster harvest in France.
Lesson Summary
• The mollusk body often has a head with eyes or tentacles, a muscular foot, a mass with organs inside, and a
mantle, which creates the external shell.
• Other mollusk structures include a gill or gills for absorbing oxygen, a complete digestive tract, and a radula.
• Mollusks are divided into ten living classes, including the familiar gastropods, cephalopods, and bivalves.
• Mollusks live in marine and freshwater habitats, as well as on land.
• Mollusks are important as food, for decoration, and in scientific studies.
Review Questions
1. What are the main characteristics of mollusks?
2. What are mollusk shells made out of?
Apply Concepts
3. What evidence shows that mollusks and annelids are related? How are they different?
4. What habitats do marine mollusks live in?
5. What makes the California sea slug ideal for studies of learning and memory?
Critical Thinking
6. Oysters, one of the bivalve filter feeders, filter up to five liters of water per hour. Oysters filter these pollutants
and either eat them or shape them into small packets that are left on the bottom where they are harmless. When there
is a high concentration of bacteria in the water from sewage run-off, this can kill the oysters and make them risky to
eat. What do you think happens to the pollutants after the bacteria enter the environment?
Further Reading / Supplemental Links
Points to Consider
• Many mollusks demonstrate bilateral symmetry. How do you think this differs from the radial symmetry
evident in echinoderms, in the next lesson?
• As we have seen, some species of mollusks live in the deep ocean around hot hydrothermal vents. In the next
lesson we will learn that many echinoderms also live in the deep sea. What adaptations do you think both
groups might have for living in such a unique environment?
• Mollusks have an exoskeleton, which is primarily external and composed of calcium carbonate. As a result
many of these are preserved in the fossil record. How do you think this compares to the type of skeleton that
an echinoderm has and to its fossil record?
Lesson Objectives
• Discuss the traits of echinoderms.
• List the types of echinoderms.
• Explain the roles echinoderms play.
Check Your Understanding
• What is meant by body symmetry?
• What is radial symmetry?
• What is bilateral symmetry?
nerve net Interconnected neurons that send signals in all directions.
water vascular system A network of fluid-filled canals; functions in gas exchange, feeding, and also in locomotion.
What are Echinoderms?
We’re all familiar with starfish (Figure 12.12 ). But sea urchins (Figure 12.13 ) and sand dollars (Figure 12.14 )
are in the same phylum. What’s similar between these three organisms? They all have radial symmetry. This means
that the body is arranged around a central point.
Echinoderms belong to the phylum Echinodermata. This phylum includes 7,000 living species. It is the largest
phylum without freshwater or land-living members.
Characteristics of Echinoderms
As mentioned earlier, echinoderms show radial symmetry. Other key echinoderm features include:
a. Despite the fact that echinoderms have a hard exterior appearance, they do not have an external skeleton.
Instead, a thin outermost skin covers an internal endoskeleton made of tiny plates and spines, contained within
tissues of the organism. This provides rigid support.
FIGURE 12.12
A starfish showing the radial symmetry
characteristic of the echinoderms.
FIGURE 12.13
Another echinoderm
a sea urchin
showing its spines.
FIGURE 12.14
An echinoderm the keyhole sand dollar.
b. Some groups, such as sea urchins, have spines (Figure 12.13 ) that protect the organism from predators and
from colonization by encrusting (covering or coating) organisms. Sea cucumbers also use these spines, or
warts, to help them move (Figure 12.15 ).
FIGURE 12.15
An echinoderm the giant California sea
3. Echinoderms have a unique water vascular system, a network of fluid-filled tubes, that helps the organism absorb
oxygen and release carbon dioxide, eat, and move. This system allows them to function without gill slits.
4. Echinoderms have a very simple digestive system, often leading directly from mouth to anus.
5. Echinoderms have an open circulatory system, meaning that fluid moves freely in the body cavity, but no heart.
6. The echinoderm nervous system is a nerve net, or interconnected neurons with no central brain.
Types of Echinoderms
The echinoderms are subdivided into two major groups:
a. Eleutherozoa, which contains the echinoderms that can move, like starfish and most other echinoderms.
b. Pelmatozoa, including immobile crinoids such as feather stars (Figure 12.16 ).
FIGURE 12.16
This passion flower feather star is an
Table 12.2 lists the four main classes of echinoderms present in the Eleutherozoa Group:
TABLE 12.2:
Echinoderm Class
Representative Organisms
Starfish and sea daisies
Brittle stars (Figure 12.17 )
Sea urchins and sand dollars
Sea cucumbers
Echinoderms are spread all over the world at almost all depths, latitudes, and environments in the ocean. Most
echinoderms are found in reefs, but some, like crinoids, live on shallow shores around the poles. In the deep ocean,
sea cucumbers are common, sometimes making up 90% of organisms.
While almost all echinoderms live on the sea floor, some sea-lilies can swim at great speeds for brief periods of time,
and a few sea cucumbers are fully floating.
Some echinoderms find other ways of moving. For example, crinoids attach themselves to floating logs, and some
sea cucumbers move by attaching to the sides of fish.
FIGURE 12.17
The giant red brittle star an ophiuroid
How Do Echinoderms Reproduce?
In most species, eggs and sperm cells are released into open water, where fertilization takes place when the eggs and
sperm meet. The release of sperm and eggs is often occurs when organisms are in the same place at the same time.
Internal fertilization takes place in a few species. Some species even take care of their offspring, like parents!
Many echinoderms have amazing powers of regeneration. Some sea stars are capable of regenerating lost arms, and
in some cases, lost arms have been observed to regenerate a second complete sea star! Sea cucumbers often release
parts of their internal organs if they perceive danger. The released organs and tissues are then quickly regenerated.
How Do Echinoderms Eat?
Feeding strategies vary greatly among the different groups of echinoderms. Different eating-methods include:
a. Passive filter-feeders, which are organisms that absorb suspended nutrients from passing water.
b. Grazers, which are organisms that feed on available plants.
c. Deposit feeders, which are organisms that feed on small pieces of organic matter, usually in the top layer of
d. Active hunters, which actively hunt their prey.
Ecological Role
Echinoderms play numerous ecological roles. Most bare rock is covered in mussels and barnacles, but the echinoderm sea urchin creates a more complex ecosystem when it also lives on the rock. Sand dollars and sea cucumbers
that burrow into the sand provides more oxygen at greater depths of the sea floor. This allows more organisms
to live there. In addition, starfish prevent the growth of algae on coral reefs, so the coral can have an easier time
Echinoderms are also the staple diet of many organisms, including the sea otter. Many sea cucumbers provide a
habitat for parasites, including crabs, worms, and snails.
Recently, some marine ecosystems have been overrun by seaweed, which caused the destruction of entire reefs.
Scientists believe that the extinction of large quantities of echinoderms caused the destruction.
Echinoderms as Food
In some countries, echinoderms are considered delicacies. Around 50,000 tons of sea urchins are captured each year,
and consumed mostly in Japan, Peru, and France. Sea cucumbers are considered a delicacy in some southeastern
Asian countries.
Echinoderms as Medicine
Echinoderms are also used as medicines, and in scientific research. For example, some sea cucumber toxins slow
down the growth rate of tumor cells, so there is an interest in using these in cancer research.
Echinoderms in Farming
The external covering of echinoderms is used as a source of lime by farmers in some areas where limestone is
unavailable. About 4,000 tons of the animals are used each year for this purpose.
Lesson Summary
• Echinoderms belong to the phylum Echinodermata, the largest phylum without freshwater or land-living members.
• Echinoderms show radial symmetry and have an endoskeleton, and a unique water vascular system. Some
have spines.
• Fertilization is generally external, and regeneration is fairly common among echinoderms.
• Echinoderms consist of two main subdivisions: the mobile Eleutherozoa and the immobile Pelmatazoa.
• Echinoderms are found all over the world at almost all depths, latitudes, and marine environments.
• Echinoderms play an important ecological and ecnomical roles in the community.
Review Questions
1. What are three important characteristics of echinoderms?
2. What three different feeding strategies of echinoderms?
3. What protection do echinoderms have against predators?
Apply Concepts
4. Chemical elements within their skeletons makes echinoderms stronger. How could this be an advantage in
echinoderms that must move to find food?
Critical Thinking
5. The larvae of many echinoderms, especially starfish and sea urchins, are pelagic, meaning they live in the open
ocean. How does being pelagic allow echinoderms to be found throughout the entire world?
Further Reading / Supplemental Links
Points to Consider
Next we discuss arthropods, which have more advanced tissues and organs.
• Why might there be an advantage to having a heart as part of the circulatory system?
Lesson Objectives
Define arthropods.
Describe the features of crustaceans.
Describe the characteristics of centipedes and millipedes.
List the features of arachnids.
Explain why arthropods are important.
Check Your Understanding
• What is an invertebrate?
• What do mollusks and echinoderms have in common?
carapace The thick dorsal shield seen in many crustaceans; often forms a protective chamber for the gills.
cephalothorax The anterior part of the arachnid body, derived from the fusion of the head and thorax.
ganglia A compact group of nerve cells having a specific function.
gastric mill A gizzard-like structure for grinding food.
molting The process by which arthropods shed their hard exoskeleton in order to grow.
pedipalps The second pair of arachnid appendages used for feeding, locomotion, and/or reproductive functions.
silk A thin, strong, protein strand extruded from the spinnerets; most commonly found on the end of the abdomen
of spiders.
What are Arthropods?
Have you ever seen an ant? A spider? A fly? A moth? With over a million described species in the phylum containing
arthropods, chances are you encounter one of these organisms every day, even without leaving your house. They
may be pests to some people, but arthropods play beneficial roles in ecosystems and economically for humans.
Arthropods belong to the phylum Arthropoda, which means “jointed feet,” and includes four living subphyla. These
Chelicerata, including spiders (Figure 12.18 ), mites, scorpions (Figure 12.19 ) and related organisms.
Myriapoda, including centipedes (Figure 12.20 ) and millipedes (Figure 12.21 ) and their relatives.
Hexapoda, including insects and three small orders of insect-like animals.
Crustacea, including lobsters (Figure 12.22 ), crabs (Figure 12.23 ), barnacles (Figure 12.24 ), crayfish
(Figure 12.25 ), and shrimp.
FIGURE 12.18
A species of spider in its web.
FIGURE 12.19
A species of scorpion.
Characteristics of Arthropods
Characteristics of arthropods include:
A segmented body with appendages on at least one segment.
Appendages that are used for feeding, sensory reception, defense, and locomotion.
A nervous system.
A hard exoskeleton made of chitin, which gives them physical protection and resistance to drying out. In order
to grow, arthropods shed this covering in a process called molting.
FIGURE 12.20
A centipede from the subphyla of myriapods.
FIGURE 12.21
A species of millipede found in Hawaii.
FIGURE 12.22
The blue American lobster illustrates
the segmented body plan of the arthropods.
FIGURE 12.23
Giant spider crabs.
FIGURE 12.24
The sessile barnacles shown here feeding.
FIGURE 12.25
A crayfish.
e. An open circulatory system with haemolymph, a blood-like fluid. A series of hearts (more than one) move the
haemolymph into the body cavity where it comes in direct contact with the tissues.
f. A complete digestive system with a mouth and anus.
g. Both aquatic and land-living arthropods have gas exchange or breathing systems. Aquatic arthropods use gills
to exchange gases. These gills have a large surface area in contact with the water, so they can absorb more
h. Land-living arthropods have internal surfaces that help exchange gasses. Insects and most other terrestrial
species have a tracheal system, where air sacs lead into the body from pores in the exoskeleton. Others use
book lungs, or gills modified for breathing air, as seen in species like the coconut crab. Some areas of the
legs of soldier crabs are covered with an oxygen absorbing skin. Land crabs sometimes have two different
structures: one that used for breathing underwater, and another used to absorb oxygen from the air.
How Many Species?
It is the largest phylum in the animal kingdom, with more than a million described species, making up more than
80% of all described living species. Arthropods are found commonly in marine, freshwater, land, and even air environments. They range in size from microscopic plankton (approximately 14 mm) up to the largest living arthropod,
the Japanese spider crab, with a leg span up to 12 feet.
Crustaceans are a large group of arthropods, consisting of almost 52,000 species. The majority of crustaceans are
aquatic, living in either ocean or freshwater habitats. A few groups have adapted to living on land, such as land crabs,
hermit crabs, and woodlice (Figure 12.26 ). Crustaceans are among the most successful animals and are found as
much in the oceans as insects are on land.
FIGURE 12.26
A terrestrial arthropod a species of
Classes of Crustaceans
Six classes of crustaceans are generally recognized. These are listed in Table 12.3 .
TABLE 12.3:
Includes brine shrimp
A small class or organisms found in
deep caves connected to salt water
The horseshoe shrimp
Includes barnacles and copepods
Small animals with bivalve shells
The largest class, with the largest
and most familiar animals: crabs,
lobsters, shrimp, krill, and woodlice
Can Crustaceans Move?
The majority of crustaceans can move, although a few groups are parasitic and live attached to their hosts. Adult
barnacles cannot move, so they attach themselves headfirst to a rock or log.
Characteristics of Crustaceans
Characteristics of crustaceans include:
a. An exoskeleton that may be bound together, such as in the carapace, the thick back shield seen in many
crustaceans that often forms a protective space for the gills.
b. A main body cavity with an expanded circulatory system. Blood is pumped by a heart located near the back.
c. A digestive system consisting of a straight tube that has a gastric mill for grinding food, and a pair of digestive
glands that absorb food.
d. Structures that function like kidneys to remove wastes. These are located near the antennae.
e. A brain that exists in the form of ganglia, or connections between nerve cells, close to the antennae and a
collection of major ganglia below the gut.
f. Crustaceans periodically shed the outer skeleton, grow rapidly for a short time, and then form another hard
skeleton. They cannot grow underneath their outer exoskeleton.
Crustaceans Reproduction
Most crustaceans have separate sexes, so they reproduce sexually using eggs and sperm. Many land crustaceans,
such as the Christmas Island red crab, mate every season and return to the sea to release the eggs. Others, such as
woodlice, lay their eggs on land when the environment is damp. In other crustaceans, the females keep the eggs until
they hatch into free-swimming larvae.
Centipedes and Millipedes
Centipedes and millipedes belong to the subphylum Myriapoda, which contains 13,000 species, all of which live
on land and are divided among four classes, (1) centipedes, (2) millipedes, (3) Symphyla, and (4) Pauropods. They
range from having over 750 legs (a species of millipede) to having fewer than ten legs. They have a single pair of
antennae and simple eyes.
The Myriapoda Habitat
Myriapoda are mostly found in moist forests, where they help to break down decaying plant material. A few live in
grasslands, semi-arid habitats, or even deserts. The majority are herbivores, but centipedes are nighttime predators.
They roam around looking for small animals to bite and eat. They eat insects, spiders, and other small invertebrates.
If the centipede is large enough it will even attack small vertebrates, like lizards. Although not generally considered
dangerous to humans, many from this group can cause temporary blistering and discoloration of the skin.
Centipedes (Figure 12.27 ) are fast, predatory, and venomous. There are around 3,300 described species, ranging
from one tiny species (less than half an inch in length) to one giant species, which may grow larger than 12 inches.
Learn more about centipedes at
FIGURE 12.27
Most millipedes are slower than centipedes and feed on leaf litter and loose organic material. Around 8,000 species
have been described, although there may be as many as 80,000 or more species actually alive.
Symphyla and Pauropods
The third class, Symphyla, contains 200 species. They resemble centipedes but are smaller and translucent. Many
spend their lives in the soil, but some live in trees. The pauropods are typically 0.5-2.0mm long and live in the soil of
all continents except Antarctica. Over 700 species have been described, and they are believed to be closely related
to millipedes.
Arachnids are a class of joint-legged invertebrates in the subphylum Chelicerata. They live mainly on land, but are
also found in freshwater and in all marine environments, except for the open ocean. There are over 100,000 named
species, including spiders, scorpions, daddy-long-legs, ticks, and mites (Figure 12.28 , Figure 12.29 , and Figure
12.30 ). There may be up to 600,000 species in total, including unknown ones.
FIGURE 12.28
A daddy-long-legs with a captured
FIGURE 12.29
Various diseases are caused by species
of bacteria that are spread to humans by
&#8220 hard&#8221 ticks like the one
shown here.
Characteristics of Arachnids
Arachnids have the following characteristics:
a. Four pairs of legs (eight total). You can tell the difference between an arachnid and an insect because insects
have three pairs of legs (six total).
FIGURE 12.30
A female crab spider sharing its flower
with velvet mites.
b. Arachnids also have two additional pairs of appendages. The first pair, the chelicerae, serve in feeding and
defense. The next pair, the pedipalps, help the organisms feed, move, and reproduce.
c. Arachnids do not have antennae or wings.
d. The arachnid body is organized into the cephalothorax, a fusion of the head and thorax, and the abdomen.
e. To adapt to living on land, arachnids have internal breathing systems, like a trachea or a book lung.
f. Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals.
g. Several groups are venomous. They release the venom from specialized glands to kill prey or enemies. Several
mites are parasitic and some of those are carriers of disease.
h. Arachnids usually lay eggs, which hatch into immature arachnids that are similar to adults. Scorpions, however, give birth to live young.
Arachnid Subgroups
The arachnids are divided into eleven subgroups. Table 12.4 shows the four most familiar subgroups, with a description of each.
TABLE 12.4: Subgroup of Arachnid
Subgroup of Arachnid
Representative Organisms
Approximate Number of
Found all over the world,
ranging from tropics to
the Arctic, some in extreme environments; All
produce silk, used for
trapping insects in webs,
aiding in climbing, forming smooth walls for burrows, producing egg sacs,
and wrapping prey Nearly
all spiders inject venom to
protect themselves or to
kill prey; only about 200
species have bites that can
be harmful to humans
TABLE 12.4: (continued)
Subgroup of Arachnid
Representative Organisms
Approximate Number of
Known for extremely long
walking legs; no silk nor
poison glands Many are
omnivores, eating small
insects, plant material and
fungi; some are scavengers, eating decaying
animal and other matter
Mostly nocturnal (come
out at night), colored in
hues of brown; a number
of diurnal (come out during the day) species have
vivid patterns of yellow,
green, and black
Characterized by a tail
with six segments, the last
bearing a pair of venom
glands and a venominjecting barb Predators
of small arthropods and
insects, they use pincers
to catch prey, then either
crush it or inject it with
a fast-acting venom,
which is used to kill or
paralyze the prey; only a
few species are harmful
to humans Nocturnal;
during the day find shelter
in holes or under rocks
Unlike the majority of
arachnids, scorpions produce live young, which
are carried about on the
mother’s back until they
have molted at least once;
they reach an age of
between four to 25 years
TABLE 12.4: (continued)
Subgroup of Arachnid
Representative Organisms
Mites and ticks
Approximate Number of
Most are small (no more
than 1.0 mm in length),
but some ticks and one
species of mite may
become 10-20 mm in
length Live in nearly
every habitat, including
aquatic and terrestrial
Many are parasitic, affecting both invertebrates
and vertebrates, and may
transit diseases to humans
and other mammals; those
that feed on plants may
damage crops
Why Arthropods are Important
Arthropods as Food
Many species of crustaceans, especially crabs, lobsters, shrimp, prawns, and crayfish, are consumed by humans.
Nearly 10,000,000 tons of arthropods as food were produced in 2005. Over 70% by weight of all crustaceans caught
for consumption are shrimp and prawns, and over 80% is produced in Asia, with China producing nearly half the
world’s total.
Arthropods in Pest Control
Humans use mites to prey on unwanted arthropods on farms or in homes. Other arthropods are used to control
weed growth. Populations of whip scorpions added to an environment can limit the populations of cockroaches and
Ecological Roles
Mites, ticks, centipedes, and millipedes are decomposers, meaning they break down dead plants and animals and
turn them into soil nutrients. This is an extremely important role because it supplies the plants with the minerals and
nutrients necessary for life. Plants pass along those minerals and nutrients to animals.
Lesson Summary
• The phylum Arthropoda includes four living subphyla: chelicerates, including spiders, mites, and scorpions;
myriapods, including centipedes and millipedes; hexapods, including insects; and crustaceans.
• Arthropods are characterized by a segmented body, appendages used for feeding, sensory structures, defense,
and locomotion, a dorsal heart, ventral nervous system, and hard exoskeleton.
• Arthropods are the largest phylum in the animal kingdom with more than a million described species; they are
found in all environments.
• Crustaceans consist of almost 52,000 species, the majority of which are aquatic.
• Arachnids mainly live on land and comprise over 100,000 named species. Adaptations for life on land include
specialized breathing structures and appendages for movement.
• Arthropods are used for food, in pest and weed control, and as decomposers, enriching the soil.
Review Questions
1. What are arthropod appendages used for?
2. What breathing systems do land-living arthropods use?
Apply Concepts
3. How is the scorpions’ method of producing young different from most other arachnids?
4. How are arthropods useful? Pick one example and explain how they are useful to humans or other organisms.
Critical Thinking
5. Arachnids have several adaptations for living on land. Pick one adaptation and explain how it is beneficial for a
land-living existence.
Further Reading / Supplemental Links
Points to Consider
Insects are the focus of the next lesson.
• Arthropods are characterized by the possession of a segmented body with appendages on at least one segment
and they are covered by a hard exoskeleton made of chitin. How do you think the general arthropod body plan
is specialized in insects?
• Insects are the only group of invertebrates to have developed flight. Compare this mode of locomotion to those
discussed in the groups of arthropods already discussed. What advantages might there be to using flight for a
method of locomotion?
Lesson Objectives
Describe the characteristics of insects.
Explain how insects obtain food.
Describe reproduction and the life cycle of insects.
Explain how insects are important.
Describe how insect pests are controlled.
Check Your Understanding
• What is an arthropod?
• Is a spider an insect? Why or why not?
exocuticle The thin and waxy water resistant outer layer of the cuticle.
homing The ability of an insect to return to a single hole among many other apparently identical holes, after a long
trip or after a long time
larvae Young or non-adult insects.
metamorphosis The process by which insects transform from an immature or young insect into an adult insect.
nymphs A developmental stage of insects, where the young is usually similar to the adult.
pheromones Chemicals secreted by animals, especially insects, that influence the behavior or development of
others within the same species.
pupa Insect metamorphosis stage in which wing development begins.
spiracles Openings on the sides of the insect abdomen, through which air is taken in.
sponging The ability of an insect mouthpart to absorb liquid food.
What Are Insects?
Insects, with over a million described species, are the most diverse group of animals on Earth. They may be found
in nearly all environments on the planet. No matter where you travel, you will see organisms from this group. Adult
insects range in size from a minuscule fairy fly to a 21.9 inch-long stick insect (Figure 12.31 ).
FIGURE 12.31
A stick insect showing how well it blends
in to its environment.
Characteristics of Insects
Characteristics of Insects include:
• Segmented bodies with an exoskeleton. The outer layer of the exoskeleton is called the cuticle. It is made up
of two layers, a thin and waxy water-resistant outer layer (the exocuticle), and an inner, much thicker layer.
The exocuticle is thinner in many soft-bodied insects and especially in caterpillars (Figure 12.32 ).
• The segments of the body are organized into three distinct but joined units: a head, a thorax, and an abdomen
(see Figure 12.33 and Table 12.5 ).
TABLE 12.5: Insect Structures
A pair antennae, a pair of compound
eyes (one to three simple eyes) and
three sets of appendages that form
the mouthparts
Six segmented legs and two or four
Has most of the digestive, respiratory, excretory, and reproductive
• The nervous system is divided into a brain and a ventral nerve cord.
• Respiration occurs without lungs. Insects do have a system of internal tubes and sacs that oxygen travels
through to reach body tissues. Air is taken in through the spiracles, openings on the sides of the abdomen.
• The circulatory system is simple and consists of only a single tube with openings. The tube pulses and
circulates blood-like fluids inside the body cavity.
FIGURE 12.32
Caterpillars feeding on a host plant.
FIGURE 12.33
A diagram of a human and an insect
comparing the three main body parts
head thorax and abdomen.
• Insect movement includes flight, walking, and swimming. Insects are the only invertebrates to develop the
ability to fly and this has played an important role in their success. Insect flight is not very well understood.
Primitive insect groups use muscles that act directly to move the wings. More advanced groups have foldable
wings and their muscles act on the wall of the thorax and give power to the wings indirectly.
• Many adult insects use six legs for walking and walk in alternate triangles touching the ground. This allows
the insect to walk quickly while staying stable. A few insects have evolved to walk on the surface of the water,
especially water striders (Figure 12.34 ). A large number of insects live parts of their lives underwater. Water
beetles and water bugs have legs adapted to paddle in the water. Young dragonflies use jet propulsion, sending
water out of their back end to move.
FIGURE 12.34
A pair of water striders mating showing how water surface tension allows for
them to stand on the water.
Insects use many different senses for both communicating and receiving information. Many insects have special
sensory organs. Table 12.6 summarizes five types of communication that are used by various insects.
TABLE 12.6: Insect Communication
Types of Communication
Visual Ultraviolet wavelengths
(light most animals cannot see)
Polarized light (light most animals
cannot see)
Representative Organisms
Bees Bees Fireflies
Perceive ultraviolet wavelengths of
light (outside of what humans can
see) Detect polarized light Reproduction and Predation Some species
produce flashes to attract mates;
other species to attract prey (food).
TABLE 12.6: (continued)
Types of Communication
Sound Production Mostly by moving appendages Ultrasound clicks
Representative Organisms
Cicadas Moths Moths Some predatory and parasitic insects
Chemical Wide range of insects
have evolved chemical communication; chemicals are used to attract,
repel, or provide other kinds of information; use of scents is especially well developed in social insects.
“Dance Language”
Honey bees
Blood-sucking insects
Loudest sounds among insects;
have special muscles to produce
sounds. Predation Produced mostly
by moths to warn bats. Predation
Some nocturnal species can hear
the ultrasonic sounds of bats, which
help them avoid predators. Can detect sounds made by prey or hosts.
Antennae of males can detect
pheromones (chemicals released
by animals, especially insects, that
influence the behavior of others
within the same species) of female
moths over distances of many miles
(Figure 12.35 ).
Have special sensory structures that
can detect infrared light in order to
find their hosts.
Honey bees are the only invertebrates to have evolved this type
of communication; length of dance
represents distance to be flown.
FIGURE 12.35
A yellow-collared scape moth showing
its feathery antennae.
Insects are Social
Social insects, such as termites (Figure 12.36 ), ants, and many bees and wasps (Figure 12.37 ), are the most familiar
social species. They live together in large well-organized colonies. Only those insects which live in nests or colonies
can home. Homing means that an insect can return to a single hole among many other apparently identical holes,
after a long trip or after a long time.
A few insects migrate, such as the monarch butterfly, which flies from Mexico to North America every winter
(Figure 12.38 ).
FIGURE 12.36
Damage to this nest brings the workers
and soldiers of this social insect the termite to repair it.
Two Major Groups of Insects
Insects are divided into two major groups:
a. Wingless: Consists of two orders, the bristle tails and the silverfish.
b. Winged insects: All other orders of insects. They are named below. How many winged orders are there?
Mayflies; dragonflies and damselflies; stoneflies; webspinners; angel insects; earwigs; grasshoppers, crickets, and
katydids; stick insects; ice-crawlers and gladiators; cockroaches and termites; mantids; lice; thrips; true bugs,
aphids, and cicadas; wasps, bees, and ants; beetles; twisted-winged parasites; snakeflies; alderflies and dobsonflies;
lacewings and antlions; Scorpios and hangingflies (including fleas); true flies; caddisflies; and butterflies, moths, and
How Insects Obtain Food
Insects have different types of appendages (arms and legs) adapted for capturing and feeding on prey. They also
have special senses that help them detect prey. Insects have a wide range of mouthparts used for feeding. Examples
FIGURE 12.37
A wasp building its nest.
FIGURE 12.38
Monarch butterflies in an overwintering
• Insects like mosquitoes and aphids have special mouthparts that help them pierce and suck. Some are herbivorous, like aphids and leafhoppers, while others eat other insects, like assassin bugs and female mosquitoes.
• Examples of chewing insects include dragonflies, grasshoppers, and beetles. Some larvae have chewing
mouthparts, as in moths and butterflies (Figure 12.39 ).
• Some insects use siphoning, as if sucking through a straw, like moths and butterflies. You may have seen a
butterfly or moth putting a long mouth-tube into at a flower while it siphons the nectar of the flower.
• Some moths, however, have no mouthparts at all.
• Some insects obtain food by sponging, like the housefly. Sponging means that the mouthpart can absorb
liquid food and send it to the esophagus. The housefly is able to eat solid food by releasing saliva and dabbing
it over the food. As the saliva dissolves the food, the sponging mouthpart absorbs the liquid food.
FIGURE 12.39
The mouth of a butterfly up close.
Reproduction and Life Cycle of Insects
Most insects can reproduce very quickly within a short period of time. With a short generation time, they evolve
faster and can adjust to environmental changes faster. Most insects reproduce by sexual reproduction. The female
produces eggs, which are fertilized by the male, and then the eggs are usually placed in a precise microhabitat at or
near the required food. Most insects are oviviparous, where the young hatch after the eggs have been laid. In some
insects, there is asexual reproduction. In the most common type, the offspring are almost identical to the mother.
This is most often seen in aphids and scale insects.
Three Types of Metamorphosis
An insect can have one of three types of metamorphosis and life cycle (Table 12.7 ). Metamorphosis describes how
insects transform from an immature or young insect into an adult insect (in at least two stages).
TABLE 12.7:
Type of Metamorphosis
Only difference between
adult and larvae (young
or non-adult insects) is
• Young,
nymphs (Figure
12.40 ), usually
similar to adult
• Wings then appear
as buds on nymphs
or early forms
• When last molt is
completed, wings
expand to full adult
• Insects have different forms in
immature and adult
stages, have different
and live in different
• Immature form is
called larvae and
remains similar in
form but increases
in size
• Larvae usually have
mouthparts even if adult
sucking ones
• At last larval stage
insect forms into
pupa (Figure 12.41
), and does not eat
or move
• During pupa stage,
wing development
begins, after which
the adult emerges
Butterflies and Moths
Importance of Insects
Many insects are considered to be pests by humans. At the same time, insects are very important.
Ecological Importance
In the environment, some insects pollinate flowering plants, as in wasps, bees, butterflies, and ants. Many insects,
especially beetles, are scavengers, feeding on dead animals and fallen trees. As decomposers, insects help create top
soil, the nutrient-rich layer of soil that helps plants grow.
FIGURE 12.40
Heteroptera nymphs and egg cases.
FIGURE 12.41
The chrysalis pupal stage of a monarch
Economic Importance
Insects also produce useful substances, such as honey, wax, lacquer, and silk. Honeybees have been raised by
humans for thousands of years for honey. The silkworm has greatly affected human history. When the Chinese used
worms to develop silk, the silk trade connected China to the rest of the world. Adult insects, such as crickets and
insect larvae, are also commonly used as fishing bait.
Insects as Food
In some parts of the world, insects are used for human food. Some people support this idea to provide a source of
protein in human nutrition. From South America to Japan, people eat roasted insects, like grasshoppers or beetles
(Figure 12.42 ).
FIGURE 12.42
Grasshoppers have been added to this
Insects in Medicine
In the past, fly larvae (maggots) were used to treat wounds to prevent or stop gangrene. Gangrene is caused by
infection of dead flesh. Maggots only eat dead flesh, so when they are placed on the dead flesh of humans, they
actually clean the wound and can prevent infection. Some hospitals still use this type of treatment.
Controlling Insect Pests
Though insects can be important, some are also considered pests. Common insect pests include:
Parasitic insects (mosquitoes, lice, bed bugs).
Insects that transmit diseases (mosquitoes, flies).
Insects that damage structures (termites).
Insects that destroy crops (locusts, weevils).
Many scientists who study insects are involved in various forms of pest control, often using insect-killing chemicals,
but more and more rely on other methods. Ways to control insect pests are described below.
• Biological control of pests in farming is a method of controlling pests by using other insects. Insect predators,
such as lady beetles and lacewings, consume a large number of other insects during their lifetime. If you add
ladybugs to your farm or garden, then they will act like a pesticide, or insect-killing chemical.
• Insecticides are most often used to kill insects. Insecticides are chemicals that kill insects. The U.S. spends $9
billion each year on pesticides. Disadvantages to using pesticides include human poisonings, killing of fish,
honeybee poisonings, and the contamination of meat and dairy products.
Lesson Summary
• Insects are the most diverse group of animals on Earth.
• They have segmented bodies with an exoskeleton, and relatively simple nervous, respiratory, and circulatory
• Insects are the only invertebrates to have developed flight.
• Some insects, like termites, ants, and many bees and wasps, are social and live together in large well-organized
• Insect movement includes flight, walking, and swimming.
• There are two major groups of insects, the wingless and the winged, and these are subdivided into various
• Insects obtain food with the use of specialized appendages for capturing and eating.
• Most insects can rapidly reproduce within a short period of time.
• An insect can have one of three types of metamorphosis and life cycle.
• Insects are beneficial both environmentally and economically.
• Insect pests can be controlled with chemical or biological means.
Review Questions
1. What are the three main parts of the insect body?
2. Why is the insect’s circulatory system said to be "simple"?
3. What does metamorphosis mean?
Apply Concepts
4. Describe the difference between the life cycle of a silverfish and the life cycle of a butterfly.
5. What makes parasitoids especially effective at killing other insect pests?
6. Describe what it means when an insect is said to be "social."
Critical Thinking
7. Why do you think an exoskeleton allowed insects to better adapt to their environments than some other invertebrates?
Further Reading / Supplemental Links
Points to Consider
Next we begin the discussion of fishes, amphibians, and reptiles.
• Some of the adaptations that insects have evolved for life on land are also displayed in amphibians and reptiles.
What could be some of these? How are they similar and different?
• Insects have some very specialized sensory capabilities. How do you think these compare to those found in
fish, amphibians, and reptiles?
MS Fishes, Amphibians, and
13.2 F ISHES
Vertebrates have a backbone. But there are many organisms that have backbones, including fish, amphibians, reptiles,
birds, and mammals! So, how do we distinguish between them? We put them in categories.
But sometimes it is difficult to put organisms into specific categories. Observe the above image. What kind of
organism is it? A snake? A lizard? A newt? It looks like a snake because it does not have legs. But it is also has the
head of a lizard. What other information do you need to know before you can classify the above organism? What
questions do you need to ask?
You may ask: Does it have ears? What other organisms have similar hearts or lungs? Are the organism’s genes more
like those of lizards or to those of snakes? Do they live in water, on land, or both?
These are the type of questions scientists ask when they encounter an organism not easily classified. Consider the
differences amongst organisms who are closely related as you read about vertebrate creatures. And then see if you
can identify the above organism.
Introduction to Vertebrates
Lesson Objectives
Describe the general features of chordates.
List the three groups of chordates and their characteristics.
List the general features of vertebrates.
Describe the classification of vertebrates.
Check Your Understanding
• What is a invertebrate?
• What is a vertebrate?
cranium a braincase
endostyle Used to gather food particles and move them along the digestive tract.
notochord A hollow nerve cord along the back.
Did you know that fish, amphibians, reptiles, birds and mammals are all related? They are all chordates. Chordates
are a group of animals that includes vertebrates, as well as several closely related invertebrates. All chordates
(phylum Chordata) have a notochord, a hollow nerve cord along the back.
They also have:
a. Pharyngeal slits, which help to filter out food particles.
b. An endostyle, which has small hairs and is used to gather food particles and move them along the digestive
c. A post-anal tail, which is present during the lifetimes of some chordates and during the development of others.
The chordate phylum is broken down into three subphyla:
a. Urochordata (represented by tunicates): Urochordates have a notochord and nerve cord only during the larval
stage (Figure ?? ). The urochordates consist of 3,000 species of tunicates, sessile marine animals with sacklike bodies and tubes for water movement.
b. Cephalochordata (represented by lancelets): Cephalochordates have a notochord and nerve cord but no vertebrae, or bones in the backbone (Figure ?? ). Cephalochordates consist of 30 species of lancelets (burrowing
marine animals).
c. Vertebrata (the vertebrates): Humans fall in this category. In all vertebrates except for hagfish, the notochord
is smaller and surrounded by vertebrae. Vertebrates all have backbones or spinal columns. About 58,000
species have been described, including many familiar groups of large land animals.
The origin of chordates is currently unknown. The first clearly identifiable chordates appear in the Cambrian Period
(about 542 - 488 million years ago) as lancelet-like specimens.
What are Vertebrates?
Vertebrates, in the subphylum Vertebrata, are chordates with a backbone. Vertebrates have a braincase, or cranium,
and an internal skeleton (except for lampreys). You can tell the difference between verebrates and other chordates
by looking at their head. Vertebrates have cephalization. Cephalization means an organism’s nervous tissue is found
toward one end of the organism. In other words, this is like having eyes in your head. Why do you think this type of
body design is an advantage?
Typical vertebrate traits include:
A backbone or spinal column.
Internal skeleton.
Defined head with pronounced cephalization.
Sensory organs, especially eyes.
Living vertebrates range in size from a carp species (Figure 13.1 ), as little as 0.3 inches, to the blue whale, as large
as 110 feet (Figure 13.2 ).
A species of carp.
An image of the blue whale a mammal
the largest living vertebrate reaching up
to 110 feet long. Shown below it is the
smallest whale species Hectorś dolphin
about 5 f eet in length and beside it a
Classification of Vertebrates
Vertebrates can be divided into two major groups: those with or without jaws. There are more than 100 species of
jawless vertebrate. There are more than 50,000 species of vertebrate with jaws.
The jawed vertebrates include species of fish with cartilage, the strong, flexible tissue found in human ears, bony
fish, and four-limbed animals. These animals are known as tetrapods. Some tetrapods include amphibians, reptiles,
birds, and mammals (Table 13.1 ).
TABLE 13.1: Species of the Main Groups of Tetrapods
Type of Tetrapod
Mammal-like Reptiles
Number of Species
Lesson Summary
Chordates are characterized by a notochord.
There are three main groups of chordates, including tunicates, lancelets and vertebrates.
Vertebrates are distinguished by having a backbone or spinal column.
Vertebrates are classified into two major groups: those without jaws and those with jaws.
Review Questions
1. What is the main feature that characterizes the chordates?
2. What are the main features of vertebrates?
Apply Concepts
two structures that
chordates possess sometime during their life cycle (during development or otherwise)
are used for food gathering, and how are these structures used?
Further Reading / Supplemental Links
Points to Consider
• How do you think a notochord could help fish adapt to swimming?
• How do you think cephalization could be an advantage in movement and feeding in fish?
Lesson Objectives
List the general traits of fish.
Describe the features of jawless fish.
List the general features of the cartilaginous fish.
Describe the features of bony fish and the significance of this superclass.
List some of the reasons why fish are important.
Check Your Understanding
• What are the unique characteristics of vertebrates?
• What are the two main groups of vertebrates?
aquaculture The raising of aquatic plants and animals, especially seaweed, shellfish and other fish.
barbels A thin fleshy structure on the external part of the head, such as the jaw, mouth or nostrils, of certain fishes.
cartilaginous skeleton A skeleton made of bone-like material called cartilage.
ectothermic cold-blooded; temperature depends on the temperature of their environment.
pineal eye An eye-like structure that develops in some cold-blooded vertebrates.
placoid Plate-like, as in the scales of sharks.
Characteristics of Fish
What exactly is a fish? You probably think the answer is obvious. You may say that a fish is an animal that swims in
the ocean or a lake. Fish are aquatic vertebrates, which became a dominant form of sea life and eventually evolved
into land vertebrates.
They have a number of characteristic traits and are classified into two major groups: jawless fish and jawed fish.
Jawed fish are further divided into those with bones and those with just cartilage. Fish, in general, are important to
humans in many ways. Can you think of some of these ways?
Some characteristics of fish include:
13.2. FISHES
a. They are ectothermic, meaning their temperature depends on the temperature of their environment. This is
unlike humans, whose temperature is controlled inside of the body.
b. They are covered with scales.
c. They have two sets of paired fins and several unpaired fins.
d. Fish also have a streamlined body that allows them to swim rapidly (Figure 13.3 ).
The humphead or Napoleon wrasse
shows some of the general traits of fish
including scales fins and a streamlined
How do Fish Breathe?
In order to absorb oxygen from the water, fish use gills (Figure 13.4 ). Gills take dissolved oxygen from water as
the water flows over the surface of the gill.
Gills help a fish breathe.
How Do Fish Reproduce?
Fish reproduce sexually. They lay eggs that can be fertilized either inside or outside the body. In most fish, the eggs
develop outside the mother’s body. In the majority of these species, fertilization takes place outside the mother’s
body. The male and female fish release their gametes into the surrounding water, where fertilization occurs. Female
fish release very high numbers of eggs to increase the chances of fertilization.
How Big Are Fish?
Fish range in size from the 51-foot whale shark (Figure 13.5 ) to the stout infantfish, which is about 0.5 inches.
One of two male whale sharks at the
Georgia Aquarium. Whale sharks are
the largest cartilaginous fish.
Exceptions to Common Fish Traits
There are exceptions to many of these fish traits. For example, tuna, swordfish, and some species of shark show
some warm-blooded adaptations, and are able to raise their body temperature significantly above that of the water
around them.
Some species of fish have a slower, more maneuverable, swimming style, like eels and rays (Figure 13.6 ). Body
shape and the arrangement of fins are highly variable, and the surface of the skin may be naked, as in moray eels, or
covered with scales. Scales can be of a variety of different types.
Although most fish live in aquatic habitats, such as ocean, lakes, and rivers, there are some that spend a lot of time
out of water. Mudskippers, for example, feed and interact with each other on mudflats for up to several days at a
time and only go underwater when digging burrows (Figure 13.7 ). They breathe by absorbing oxygen across the
skin, similar to how frogs breathe.
See;feature=channel for further information (5:35).
Click image to the left for more content.
13.2. FISHES
One of the cartilaginous fish a stingray
shows very flexible pectoral fins connected to the head.
A mudskipper shown on the mudflats
where it spends time feeding and interacting with other individuals.
Agnatha: Jawless Fishes
Jawless fish, part of the class Agnatha, belong to the phylum Chordata, subphylum Vertebrata. There are two living
groups of jawless fish, with about 100 species in total: lampreys and hagfish (Figure 13.8 ). Although hagfish belong
to the subphylum Vertebrata, they do not technically have vertebrae.
A hagfish
In addition to the absence of jaws, fish in this class are characterized by the absence of paired fins or an identifiable
stomach. Characteristics they do have include:
a. A notochord, both in larvae and adults.
b. Seven or more paired gill pouches.
c. The branchial arches, a series of arches that support the gills of aquatic amphibians and fishes, lie close to the
body surface.
d. A light sensitive pineal eye, an eye-like structure that develops in some cold-blooded vertebrates.
e. A cartilaginous skeleton, or a skeleton made of bone-like material called cartilage.
f. A heart with two chambers.
g. Reproduction using external fertilization.
h. They are ectothermic.
Many jawless fish in the fossil record were armored with heavy bony-spiky plates. The first armored fish in this class
evolved before bony fish and tetrapods, including humans.
Cartilaginous Fishes
So why did fish eventually evolve to have jaws? Such an adaptation would allow fish to eat a much wider variety of
food, including plants and other organisms. The cartilaginous fishes are jawed fish with paired fins, paired nostrils,
scales, two-chambered hearts, and skeletons made of cartilage rather than bone. Cartilage does not have as much
calcium as bones, which makes bones rigid. Cartilage is softer and more flexible than bone.
The 1,000 or so species of cartilaginous fish are subdivided into two subclasses: the first includes sharks, rays, and
skates; the second includes chimaera, sometimes called ghost sharks. Fish from this group range in size from the
dwarf lanternshark, at 6.3 inches, to the 50-foot whale shark shown in Figure ?? .
13.2. FISHES
Blood, Skin, and Teeth
Since they do not have bone marrow (as they have no bones), red blood cells are produced in the spleen, in special
tissue around the gonads, and in an organ called Leydig’s Organ, only found in cartilaginous fishes. The tough skin
of this group of fish is covered with dermal teeth, or placoid scales. In adult chimaera, the placoids are very small,
making it feel like sandpaper. The teeth found in the mouth of cartilaginous fish evolved from teeth found on the
The sharks, rays and skates are further broken into two superorders:
a. Rays and skates.
b. Sharks (Figure 13.9 ).
Sharks are some of the most frequently studied cartilaginous fish. Sharks are distinguished by such features as:
The number of gill slits.
The numbers and types of fins.
The type of teeth.
Body shape.
Their activity at night.
An elongated, toothed snout used for slashing the fish that they eat, as seen in sawsharks.
Teeth used for grasping and crushing shellfish, a characteristic of bullhead sharks.
A whisker-like organ named barbels, a characteristic of carpet sharks.
A long snout (or nose-like area), characteristic of groundsharks.
Large jaws and ovoviviparous reproduction, where the eggs develop inside the mother’s body after internal
fertilization, and the young are born alive. This trait is characteristic of mackerel sharks.
A spotted Wobbegong shark at Shelly
Beach Sydney Australia showing skin
flaps around the mouth and camouflage
Bony Fishes
There are almost 27,000 species of bony fish, which are divided into two classes: ray-finned fish and lobe-finned fish.
Most bony fish are ray-finned. There are only eight living species of lobe-finned fish, including lungfish (Figure
13.10 ) and coelacanths (Figure 13.11 ).
FIGURE 13.10
One of the only eight living species of
lobe-finned fish the lungfish.
FIGURE 13.11
One of the eight living species of lobe
finned fish the coelacanth.
Most fish are bony fish, making them the largest group of vertebrates in existence today. They are characterized by:
a. A head and pectoral girdles (arches supporting the forelimbs) that are covered with bones derived from the
13.2. FISHES
b. A lung or swim bladder, which helps the body create a balance between sinking and floating by either filling
up with or emitting gases such as oxygen
c. Jointed, segmented rods supporting the fins.
d. A cover over the gill, which helps them breathe without having to swim.
e. The ability to see in color, unlike most other fish.
One of the most interesting adaptations of these fish is their ability to produce replacement, bone, by replacing
cartilage from within, with bone. They also produce “spongy," bone. This means that the fish have a lightweight,
flexible bone on the inside, surrounded by stronger and more rigid bone on the outside. This type of bone allows the
fish to move in different ways compared to the cartilaginous fish.
How Big Are Bony Fish?
The ocean sunfish is the most massive bony fish in the world, up to 11 feet long, and weighing up to 5,070 pounds
(Figure 13.12 ). Other very large bony fish include the Atlantic blue marlin, the black marlin, some sturgeon species,
the giant grouper and the goliath grouper. In contrast, the dwarf pygmy goby measures only 0.6 inches.
FIGURE 13.12
An ocean sunfish the most massive
bony fish in the world up to 11 feet long
and weighing 5 070 pounds
Why Fish are Important
How are fish important? Of course, they are used as food (Figure 13.13 ). In fact, people all over the world
either catch fish in the wild or farm them in much the same way as cattle or chickens, a type of farming known as
aquaculture. Fish are also caught for recreation to display in the home and or in public aquaria.
FIGURE 13.13
Workers harvest catfish from the Delta
Pride Catfish farms in Mississippi.
Lesson Summary
• The general traits of fish help adapt them for living in an aquatic environment, mostly for swimming, and also
for absorbing oxygen.
• Fish are ectothermic (cold-blooded), although some show warm-blooded adaptations.
• Jawless fish do not have bone, but they do have cartilage.
• Fish with jaws consist of both the cartilaginous fish and the bony fish.
• Cartilaginous fishes include sharks, rays, skates and chimaera.
• Bony fish form the largest group of vertebrates in existence today, and have true bone that can regenerate.
• Fish are an important food source for humans.
Review Questions
1. What are three traits that all fish have in common?
2. What is an example of one exception to the general traits of fish?
3. What are three characteristics of jawless fish?
4. List three ways that fish are important.
13.2. FISHES
Apply Concepts
5. Between the hagfish and a cartiliaginous fish, the structure of the jaw evolved? Why do you think this major
structure evolved in fish? How does it benefit them?
6. What is the major difference between cartilaginous fish and bony fish? Why do you think the bony fish evolved
to be different?
Critical Thinking
7. Mudskippers are an example of a fish species that must absorb oxygen across the skin, instead of through gills,
since they spend most of their time out of water. What kind of environment would pressure fish to evolve to breathe
in air instead of breathing in the water?
Further Reading / Supplemental Links
Points to Consider
Amphibians are next.
• How do you think the gills of fish relate to the lungs of other animals?
• Lungfish have paired lungs similar to those of tetrapods. How do you think the breathing systems of lungfish
could be similar to and different from tetrapods in the way they breathe?
• What structures are different between fish and amphibians, that allow amphibians to live on land?
Lesson Objectives
Describe amphibian traits.
List the features of salamanders.
Compare and contrast frogs and toads with other amphibians.
Describe the roles of amphibians.
Check Your Understanding
• What are some adaptations that amphibians, like fish, have for living in the water?
• What are the characteristics that amphibians share with all other vertebrates?
convergent adaptation The appearance of similar traits in groups of animals that are evolutionarily unrelated to
each other.
ecdysis The ability to regenerate lost limbs, as well as other body parts.
hyoid bone A U-shaped bone at the root of the tongue; in salamanders it is used to help catch prey.
tympanum Equivalent to the middle ear; used in hearing.
valarian respiration Respiration in which the capillary beds are spread throughout the epidermis, so that gases
can be exchanged through the skin.
Characteristics of Amphibians
What group of animals begins its life in the water, but then spends most of its life on land? Amphibians! Amphibians
are a group of vertebrates that has adapted to live in both water and on land. Their ancestors evolved from living in
the sea to living on land. There are approximately 6,000 species of amphibians, of many different body types, physiologies, and habitats, ranging from tropical to subarctic regions. Frogs, toads, salamanders, newts, and caecilians
are all types of amphibians (Figure 13.14 ).
Like fish, amphibians are ectothermic vertebrates. They belong to the class Amphibia. There are three orders:
FIGURE 13.14
One of the many species of amphibian
is this dusky salamander.
a. Urodela, containing salamanders and newts.
b. Anura, containing frogs and toads.
c. Apoda, containing caecilians.
Amphibian larvae are born and live in water, and they breathe using gills. The adults live on land for part of the time,
and breathe both through their skin and using lungs.
Where do Amphibians Live?
Most amphibians live in fresh water, not salt water. Although there are no true saltwater amphibians, a few can live
in brackish (slightly salty) water. Some species do not need any water at all, and several species have also adapted
to live in drier environments. Most amphibians still need water to lay their eggs.
How do Amphibians Reproduce?
Amphibians reproduce sexually. The life cycle of amphibians happens in the following stages:
a. Egg Stage: Amphibian eggs are fertilized in a number of ways. External fertilization, employed by most frogs
and toads, involves a male gripping a female across her back, almost as if he is squeezing the eggs out of her.
The male releases sperm over the female’s eggs as they are laid. Another method is used by salamanders,
whereby the male deposits a packet of sperm onto the ground. The female then pulls it into her cloaca,
where fertilization occurs internally. By contrast, caecilians and tailed frogs use internal fertilization, just like
reptiles, birds and mammals. The male deposits sperm directly into the female’s cloaca.
b. Larval stage: When the egg hatches, the organism is legless, lives in water and breathes with gills.
c. During the larval stage, the amphibian slowly transforms into an adult by losing its gills and growing four
legs. Once development is complete, it can live on land.
How did Amphibians Adapt to Living on Land?
In order to live on land, amphibians replaced gills with another respiratory organ, the lungs.
Other adaptations include:
• A glandular skin that prevents loss of water.
• Eyelids that allow them to adapt to vision outside of the water.
• An eardrum developed to separate the external ear from the middle ear.
• In frogs and toads, the tail disappears in adulthood.
Salamanders belong to a group of approximately 500 species of amphibians. The order Urodela, containing salamanders and newts, is divided into three suborders:
a. Giant salamanders (including the hellbender and Asiatic salamanders).
b. Advanced salamanders (including lungless salamanders, mudpuppies, and newts).
c. Sirens.
Salamanders are characterized by slender bodies, short legs, and long tails. They are most closely related to the
caecilians, little-known legless amphibians (Figure 13.15 and Figure ?? ). Since they have moist skin, salamanders
live in or near water or on moist ground, often in a swamp. Some species live in water most of their life, some live
their entire adult life on land, and some live in both habitats. Salamanders are carnivorous, eating only other animals,
not plants. They will eat almost any smaller animal. Finally, salamanders have the ability to grow back lost limbs,
as well as other body parts. This process is known as ecdysis.
FIGURE 13.15
The marbled salamander shows the typical salamander body plan slender body
short legs long tail and moist skin.
How Do Salamanders Breathe?
Different salamanders breathe in different ways. In those that have gills, breathing occurs through the gills as water
passes over the gill slits. Species that live on land have lungs that are used in breathing, much like breathing in
mammals. Other land-living salamanders do not have lungs or gills. Instead, they "breathe", or exchange gases,
through their skin. This is known as valarian respiration, and requires blood vessels that exchange gases to be
spread throughout the skin.
How Big Are Salamanders?
Salamanders are found in most moist or arid habitats in the northern hemisphere. They are generally small, but some
can reach a foot or more, as in the mudpuppy of North America. In Japan and China, the giant salamander reaches
6 feet and weighs up to 66 pounds (Figure 13.16 ).
FIGURE 13.16
The Pacific giant salamander can reach
up to 6 feet in length and weigh up to 66
Frogs and Toads
Frogs and toads (Figure 13.17 ) are amphibians in the order Anura. In terms of classification, there is actually not a
big difference between frogs and toads. Some amphibians that are called "toads" have leathery, brown colored, wart
covered skin, but they are still in the same order as frogs.
FIGURE 13.17
A toad showing typical characteristics
of leathery and warty skin and brown
Frogs are found in many areas of the world, from the tropics to subarctic regions, but most species are found in
tropical rainforests. Consisting of more than 5,000 species (about 88% of amphibian species are frogs), they are
among the most diverse groups of vertebrates. Frogs range in size from less than 0.5 inches in species in Brazil and
Cuba to 1-foot goliath frog of Cameroon.
Characteristics of Frogs
Adult frogs are characterized by long hind legs, a short body, webbed finger-like parts, and the lack of a tail (Figure
13.18 ). They also have a three-chambered heart, as do all tetrapods except birds and mammals. Most frogs live
part of the time in water and part of the time on land, and move easily on land by jumping or climbing. To become
great jumpers, frogs evolved long hind legs and long ankle bones. They also have a short backbone with only 10
vertebrae. Frog and toad skin hangs loosely on the body, and skin texture can be smooth, warty, or folded.
In order to live on land and in water, frogs have three eyelid membranes: one is see-through to protect the eyes
underwater, and two other ones let them see on land. Frogs also have a tympanum, which acts like a simple ear.
They are found on each side of the head. In some species, the tympanum is covered by skin.
FIGURE 13.18
A tree frog. Notice the powerful muscles
in the limbs and the coverings around
the eyes.
Frogs typically lay their eggs in puddles, ponds or lakes. Their larvae, or tadpoles, have gills, and the frogs develop
in water. You may hear males "ribbiting," producing a mating call used to attract females to the bodies of water best
for mating and breeding. Frogs calls can occur during the day or night.
Listen here;feature=fvw for these distinctive sounds (3:30).
Click image to the left for more content.
Adult frogs are meat-eaters and eat mostly arthropods, annelids, and gastropods. Frogs do not have teeth on their
lower jaw, so they usually swallow their food whole, using the teeth they do have to hold the prey in place. Other
frogs do not have any teeth, so they must swallow their prey whole.
Roles of Amphibians
Amphibians as Foods
Frogs are raised as a food source. Frog legs are a delicacy in China, France, the Philippines, northern Greece and
the American south, especially Louisiana.
Amphibians in Research
Amphibians are used in cloning research and other branches of embryology, because their eggs lack shells, so it is
easy to watch their development.
The African clawed frog, Xenopus laevis, is a species that is studied to understand certain biological phenomena in
developmental biology, because it is easy to raise in a lab and has a large and easy to study embryo. Many Xenopus
genes have been identified and cloned, especially those involved in development.
Many environmental scientists believe that amphibians, including frogs, indicate when an environment is damaged.
Since they live on land and water, when species of frogs begin to decline, it often indicates that there is a bigger
problem with the ecosystem.
Amphibians in Popular Culture
Amphibians can be found in folklore, fairy tales and popular culture. Numerous legends have developed over the
centuries around the salamander (its name originates from the Persian, for “fire” and “within") , many related to fire.
This connection likely originates from the tendency of many salamanders to live inside rotting logs. When placed
into the fire, salamanders would escape from the logs, lending to the belief that the salamander was created from
Lesson Summary
Amphibians have adaptations for both aquatic (gills), and terrestrial (lungs and moist skin) lifestyles.
Most amphibians must reproduce in water.
Development includes a shell-less egg, larval stage, and adult stage.
Salamanders have some unique features, including the use of the hyoid bone in hunting prey, and the process
of ecdysis.
• Adult frogs and toads have features for living in the water (such as webbed finger-like parts) and for living on
the land (such as long hind legs for jumping).
• Frogs are well known for their mating calls, which are used to attract females to aquatic breeding grounds.
• Amphibians play a role as a food source, are used in various types of biological research, can serve as indicators of ecosystem health, and are found in folklore and popular culture.
Review Questions
1. Describe three general traits of amphibians.
2. Describe the life cycle of amphibians from egg stage to adult stage.
Apply Concepts
3. What are two ways that amphibians have adapted to living on land?
4. Why would a scientist want to use a frog for research?
5. Name one way amphibians have evolved to avoid predation.
Critical Thinking
6. A frog’s skin must remain moist at all times in order for oxygen to pass through the skin and into the blood. Why
does this fact make frogs susceptible to many toxins in the environment?
Further Reading / Supplemental Links
Points to Consider
Future studies of molecular genetics should soon provide further insights to the evolutionary relationships among
frog families. These studies will also clarify relationships among families belonging to the rest of vertebrates. We
discuss reptiles next.
• Although care of offspring is poorly understood in frogs, it is estimated that up to 20% of amphibian species
care for their young, and that there is a great diversity of parental behaviors. As you begin to examine the
reproductive system of reptiles in the next lesson, think about what kinds of parental behaviors reptiles might
have and how they compare to that of amphibians.
Lesson Objectives
List reptile traits.
Describe the general features of lizards and snakes.
List the characteristics of alligators and crocodiles.
Describe the traits of turtles.
Explain the importance of reptiles.
Check Your Understanding
• How have amphibians adapted to living on land?
• What features in amphibians are also useful to reptiles who live in water?
amniotes Vertebrates whose embryos are surrounded by an amniotic membrane.
nictitating membrane A third transparent eyelid.
poikilothermic Cold-blooded; without the ability to independently warm the blood.
Traits of Reptiles
What reptiles do you know? Snakes, alligators, and crocodiles are all reptiles. Reptiles are tetrapods and amniotes,
which means their embryos are surrounded by a thin membrane. Modern reptiles live on every continent except
Antarctica. They range in size from the newly-discovered Jaragua Sphaero, at 0.6 inches, to the saltwater crocodile,
at up to 23 feet.
There are four living orders of reptiles:
Squamata, which includes lizards, snakes, and amphisbaenids (or “worm-lizards”).
Crocodilia, which include crocodiles, gharials (Figure 13.19 ), caimans, and alligators.
Testudines, which includes turtles and tortoises.
Sphenodontia, which includes tuatara (Figure 13.20 ).
Reptiles are air-breathing, ectothermic vertebrates that have skin covered in scales. Most reptiles have a closed
circulatory system with a three-chambered heart. All reptiles breathe using lungs. They also have two small kidneys.
FIGURE 13.19
An Indian gharial crocodile.
FIGURE 13.20
A tuatara.
Usually their sense organs, like ears, are well developed, though snakes do not have external ears (middle and inner
ears are present). All reptiles have advanced eyesight.
How do Reptiles Reproduce?
The majority of species are egg-laying, although certain species of squamates can give birth to live young. This
is achieved either by oviparity (the egg stays in the female until birth), or viviparity (offspring born without eggs).
Many of the viviparous species feed their fetuses by a placenta, similar to those of mammals. Some reptiles provide
care for their young.
All reptiles have a cloaca, a single exit and entrance for sperm, eggs, and waste, located at the base of the tail.
Most reptiles lay amniotic eggs covered with leathery or calcium-containing shells. An amnion (the innermost of
the embryonic membranes), chorion (the outermost of the membranes surrounding the embryo), and allantois (a
vascular embryonic membrane) are present during embryonic life. There are no larval stages of development.
Most reptiles reproduce sexually, although six families of lizards and one snake are capable of asexual reproduction.
In some species of squamates, a population of females is able to produce a nonsexual diploid clone of the mother.
This asexual reproduction, called parthenogenesis, also occurs in several species of gecko.
Lizards and Snakes
Lizards and snakes belong to the largest order of reptiles, Squamata. Lizards are a large group of reptiles, with nearly
5,000 species, living on every continent except Antarctica.
Characteristics of Squamata
Members of the order are distinguished by horny scales or shields and movable quadrate bones, which make it
possible to open the upper jaw very wide. Quadrate bones are especially visible in snakes, which are able to open
their mouths very wide to eat large prey (Figure 13.21 ).
FIGURE 13.21
A corn snake swallowing a mouse.
Characteristics of Lizards
Key features of lizards include:
Four limbs.
External ears.
Movable eyelids.
A short neck.
A long tail, which they can shed in order to escape from predators.
They eat insects.
Vision, including color vision, is well-developed in lizards. You may have seen a lizard camouflaged to blend in
with its surroundings. Since they have great vision, lizards communicate by changing the color of their bodies. They
also communicate by chemical signals called pheromones.
Adult lizards range from 1 inch in length, like some Caribbean geckos, to nearly 10 feet (Figure 13.22 ).
FIGURE 13.22
A Komodo dragon the largest of the
lizards attaining a length of 10 feet.
With 40 lizard families, there is an extremely wide range of color, appearance and size of lizards. Many lizards are
capable of regenerating lost limbs or tails. Almost all lizards are carnivorous, although most are so small that insects
are their primary prey. A few species are omnivorous or herbivorous, and others have reached sizes where they can
prey on other vertebrates, such as birds and mammals.
Lizard Behavior
Many lizards are good climbers or fast sprinters. Some can run on two feet, such as the collared lizard. Some, like
the basilisk, can even run across the surface of water to escape. Many lizards can change color in response to their
environments or in times of stress (Figure 13.23 ). The most familiar example is the chameleon, but more subtle
color changes can occur in other lizard species.
FIGURE 13.23
A species of lizard showing general
body form and camouflage against
Legless Lizards
Some lizard species, including the glass lizard and flap-footed lizards, have evolved to lose their legs, or their legs
are so small that they no longer work. Legless lizards almost look like snakes, though structures leftover from earlier
stages of evolution remain. For example, flap-footed lizards can be distinguished from snakes by their external ears.
Characteristics of Snakes
All snakes are meat-eaters, and are different from legless lizards because they do NOT have eyelids, limbs, external
ears, or forelimbs. The more than 2,700 species of snake can be found on every continent except Antarctica and
range in size from the tiny, 4-inch-long thread snake to pythons and anacondas that are over 17 feet long (Figure
13.24 ).
In order to fit inside of snakes’ narrow bodies, paired organs (such as kidneys) appear one in front of the other instead
of side by side. Snakes’ eyelids are transparent “spectacle” scales which remain permanently closed. Most snakes
are not venomous, but some have venom capable of causing painful injury or death to humans. However, snake
venom is primarily used for killing prey rather than for self-defense.
Most snakes use specialized belly scales, which grip surfaces, to move. The body scales may be smooth, keeled or
granular (Figure 13.25 ). In the shedding of scales, known as molting, the complete outer layer of skin is shed in one
layer (Figure 13.26 ). Molting replaces old and worn skin, allows the snake to grow, and helps it get rid of parasites
such as mites and ticks.
Although different snake species reproduce in different ways, all snakes use internal fertilization. The male uses sex
organs stored in its tail to transfer sperm to the female. Most species of snakes lay eggs, and most species abandon
these eggs shortly after laying.
How do Snakes Eat?
All snakes are strictly carnivorous, eating small animals including lizards, other snakes, small mammals, birds, eggs,
fish, snails or insects. Because snakes cannot bite or tear their food to pieces, prey must be swallowed whole. The
body size of a snake has a major influence on its eating habits. The snake’s jaw is unique in the animal kingdom.
FIGURE 13.24
A species of anaconda one of the
largest snakes which can be as long as
17 feet.
FIGURE 13.25
A close up of snake scales of a banded
krait showing black and yellow alternating bands and spaces between scales.
FIGURE 13.26
A northern water snake shedding its
Snakes have a very flexible lower jaw, the two halves of which are not rigidly attached. They also have many other
joints in their skull, allowing them to open their mouths wide enough to swallow their prey whole.
Some snakes have a venomous bite, which they use to kill their prey before eating it. Other snakes kill their prey by
strangling them, and still others swallow their prey whole and alive. After eating, snakes enter a resting stage, while
the process of digestion takes place. The process is highly efficient, with the snake’s digestive enzymes dissolving
and absorbing everything but the prey’s hair and claws, even when the prey is swallowed whole!
Alligators and Crocodiles
Crocodilia, containing both alligators and crocodiles, is an order of large reptiles. Reptiles belonging to Crocodilia
are the closest living relatives of birds. Reptiles and birds are the only known living descendants of the dinosaurs.
Think about how organisms with the same ancestors can evolve to be so different.
The basic crocodilian body plan (Figure 13.27 ) is a very successful one, and has changed little over time. Modern
species actually look very similar to their Cretaceous ancestors of 84 million years ago.
Characteristics of Crocodiles
Crocodilians have a flexible, semi-erect posture. They can walk in low, sprawled “belly walk,” or hold their legs
more directly underneath them to perform the “high walk.” Most other reptiles can only walk in a sprawled position.
All crocodilians have, like humans, teeth set in bony sockets, but unlike mammals, they replace their teeth throughout
life. Crocodilians also have a secondary bony palate that enables them to breathe when under water, even if the mouth
is full of water. Their internal nostrils open in the back of their throat, where a special part of the tongue called the
palatal valve closes off their respiratory system when they are underwater, allowing them to breathe.
Crocodiles and gharials (large crocodilians with longer jaws) have salivary glands on their tongue, which are used to
remove salt from their bodies. Crocodilians are often seen lying with their mouths open, a behavior called gaping.
One of its functions is probably to cool them down, but it may also have a social function.
FIGURE 13.27
crocodilian body plan.
Crocodilians are known to swallow stones, known as gastroliths, which help digest their prey. The crocodilian
stomach is divided into two chambers. The first is powerful and muscular. The other stomach is the most acidic
digestive system of any animal. It can digest mostly everything from their prey, including bones, feathers, and horns!
The sex of developing crocodilians is determined by the temperature of the eggs during incubation (eggs are kept
warm before they hatch). This means that the sex of crocodilians is not determined genetically. If the eggs are kept
at a cold or a hot temperature, then their offspring may be all male or all female. To get both male and female
offspring, the temperature must be kept within a narrow range.
Evolving More Complex Structures
Like all reptiles, crocodilians have a relatively small brain, but the crocodilian brain is more advanced than those of
other reptiles. As in many other aquatic or amphibian tetrapods, the eyes, ears, and nostrils are all located on the
same "face" in a line one after the other. They see well during the day and may even have color vision, but they have
excellent night vision. A third transparent eyelid, the nictitating membrane, protects their eyes underwater.
While birds and most reptiles have a ring of bones around each eye which supports the eyeball, crocodiles lack these
bones, just like mammals and snakes. The eardrums are located behind the eyes and are covered by a movable flap of
skin. This flap closes, along with the nostrils and eyes, when they dive, preventing water from entering their external
head openings. The middle ear cavity has a complex of bony air-filled passages and a branching tube.
The upper and lower jaws are covered with "sensory pits," which hold bundles of nerve fibers that respond to the
slightest disturbance in surface water. Crocodiles can detect vibrations and small pressure changes in water, making
it possible for them to sense prey and danger even in total darkness.
Like mammals and birds, and unlike other reptiles, crocodiles have a four-chambered heart. But, unlike mammals,
blood with and without oxygen can be mixed.
Turtles are reptiles in the order Testudines. If you have seen turtles before, what is the most noticeable thing about
them? Their shells. Most turtle bodies are covered by a special bony or cartilaginous shell developed from their ribs.
About 300 species are alive today, and some are highly endangered. Like other reptiles, turtles are poikilothermic,
meaning their temperature changes in response to their environment.
Turtles are broken down into two groups, based on how they bring their neck back into their shell:
a. Cryptodira, which can draw their neck inside and under their spine.
b. Pleurodira, which fold their necks to one side.
Characteristics of Turtles
Although many turtles spend large amounts of their lives underwater, they can also spend much of their lives on dry
land and breathe air. Turtles cannot breathe in water, but can hold their breath for long periods of time. Turtles must
surface at regular intervals to refill their lungs.
Turtles don’t lay eggs underwater. Turtles lay slightly soft and leathery eggs, like other reptiles. The eggs of the
largest species are spherical, while the eggs of the rest are longer in shape (Figure 13.28 ).
FIGURE 13.28
Turtle eggs.
Most turtles that spend most of their life on land have their eyes looking down at objects in front of them. Some
aquatic turtles, such as snapping turtles and soft-shelled turtles, have eyes closer to the top of the head. These species
of turtles can hide from predators in shallow water, where they lie entirely submerged in water except for their eyes
and nostrils.
Sea turtles (Figure 13.29 ) have glands near their eyes that produce salty tears, which remove excess salt taken in
from the water they drink.
Turtles have exceptional night vision due to the unusually large number of cells that sense light in their eyes. Turtles
have color vision.
FIGURE 13.29
A species of sea turtle showing placement of eyes shell shape and flippers.
In some species, temperature determines whether an egg develops into a male or female. Large numbers of eggs
are placed by the female in holes dug into mud or sand. They are then covered and left to grow and develop by
themselves. When the turtles hatch, they squirm their way to the surface and head toward the water.
How do Turtles Eat?
Turtles have a rigid beak and use their jaws to cut and chew food. Instead of teeth, the upper and lower jaws of the
turtle are covered by horny ridges. Carnivorous turtles usually have knife-sharp ridges for slicing through their prey.
Herbivorous turtles have serrated ridges that help them cut through tough plants.
How Big Are Turtles?
The largest turtle is the great leatherback sea turtle (Figure 13.30 ), which can have a shell length of 7 feet and can
weigh more than 2,000 pounds. The only surviving giant tortoises are on the Seychelles and Galapagos Islands and
can grow to over 4 feet in length and weigh about 670 pounds (Figure 13.31 ). The smallest turtle is the speckled
padloper tortoise of South Africa, measuring no more than 3 inches in length, and weighing about 5 ounces.
Importance of Reptiles
The chief impact of reptiles on humans is their role as predators of pest species. For example, in many different
countries, like India, snakes kill rats that can be pests and carriers of disease. Also, since turtles live for hundreds of
years, genetic researchers are examining the turtle’s DNA for possible genes involved in a long life.
Reptiles as Food
Reptiles are also important as food sources:
• Green iguanas are eaten in Central America.
FIGURE 13.30
The leatherback turtle can reach up to
7 feet in length and weigh over 2 000
FIGURE 13.31
A Galapagos giant tortoise can grow to
over 4 ft in length and weigh about 670
• The tribals of Irulas from Andhra Pradesh and Tamil Nadu in India are known to eat some of the snakes they
• Cantonese snake soup is consumed by local people in the fall to prevent colds.
• Cooked rattlesnake meat is commonly consumed in parts of the Midwestern United States.
• Turtle soup is consumed throughout the world.
Reptiles as Pets
Reptiles also make good pets. In the Western world, some snakes, especially less aggressive species, like the ball
python or corn snake, are kept as pets. Turtles, particularly small land-dwelling and freshwater turtles, are also
common pets. Among the most popular are Russian tortoises, Greek spur-thighed tortoises and terrapins.
Reptiles in Art and Culture
Finally, reptiles play a significant role in folklore, religion and popular culture. The Moche people of ancient Peru
worshipped reptiles and often put lizards in their art. Snakes or serpents are connected to healing and to the Devil.
Since snake’s shed and then heal again, they are a symbol of healing and medicine, as shown in the Rod of Asclepius
(Figure 13.32 ).
In Egyptian history, the Nile cobra is found on the crown of the pharaoh. This snake was worshipped as one of the
Lesson Summary
• Reptiles are air-breathing, cold-blooded vertebrates characterized by a scaly skin.
• Reptiles have a variety of reproductive systems, with different strategies for providing nutrition to developing
• Lizards and snakes are distinguished by a unique type of scaly skin and movable quadrate bones.
• Lizards have some unique adaptations, including regeneration of lost limbs or tails and changing color.
• Snakes are distinguished by lack of eyelids, limbs, external ears and vestiges of forelimbs.
• Snakes have various adaptations for killing and eating their prey.
• Crocodilia have a flexible semi-erect posture, lifelong replacement of teeth, and a secondary bony palate.
• The sex of developing crocodilians is determined by the incubation temperature of the eggs.
• Other crocodilian traits, such as salt glands, nictitating membranes, ear flaps, and sensory pits, are adaptations
for aquatic living.
• Turtles are characterized by a special bony or cartilaginous shell. They have specialized adaptations for aquatic
living, such as eye placement and salt glands, and adaptations for terrestrial living as well, like placement of
eyes and protection of eggs.
• Reptiles play important roles as predators of pest species, food sources, pets, in medical and scientific research,
and in folklore, religion and popular culture.
Review Questions
1. Describe three general traits of reptiles.
FIGURE 13.32
The Rod of Asclepius where the snake
is a symbol of healing and medicine.
2. Describe two different types of reproduction in reptiles.
3. How are snakes different from legless lizards?
Apply Concepts
4. Name two adaptations of a crocodilian stomach which help it in digestion.
5. Name two organs that a turtle or a snake have that closely resemble organs in humans. Why are they similar?
Critical Thinking
6. The shape and structure of a turtle’s shell can give its inhabitant advantages for avoiding predators, aid in swimming and diving, and for walking on land. Given what you know about a turtle’s shell, explain how the structure and
shape could help the turtle in the above situations.
Further Reading / Supplemental Links
Points to Consider
Next, we continue our discussions with birds and mammals
• What colorful displays do you think are used to attract mates in birds and mammals?
• How do you think the hearts of fish, amphibians, and reptiles compare to that of birds and mammals?
• How do birds or mammals reproduce? By eggs like fish and amphibians? Or do they have live births?
MS Birds and Mammals
14.1 B IRDS
Observe the above organism. It has a bill like a duck. Does that make it a bird? It has fur like a dog or beaver, so
is it a mammal? But it also has webbed feet. What is this creature? These are the questions scientists asked when
they first discovered the duck-billed platypus. It is classified as a mammal, but it also has some bird and even reptile
DNA. Surprisingly, it also lays eggs, while almost all other mammals give birth to live young.
The platypus shows how great the diversity of life is on Earth. Organisms are not always easily classifiable, but most
warm-blooded vertebrates can be classified as either a bird or a mammal.
Lesson Objectives
List and describe general traits of birds.
Explain how birds are adapted for flight.
List different breeding systems in birds and describe nesting, incubation and parental care.
Illustrate the diversity of birds with examples of some of the varied groups.
Explain how birds are important, both economically and ecologically.
Check Your Understanding
• Birds and reptiles have some traits in common. For example, birds are egg-layers and most reptiles are also
oviparous. What do the eggs of both groups have in common?
• What traits do birds have as a result of their being warm-blooded?
aerofoil A surface which is designed to aid in lifting or controlling by making use of the air currents through which
it moves.
altricial Newborn that are helpless at birth and require much parental care.
monogamous A mating system where the couple pair for the duration of the breeding season, or sometimes for a
few years or until one mate dies.
polygamous A mating system in which where there is more than one mate.
precocial Newborn that are independent at birth or hatching and require little parental care.
Characteristics of Birds
How many different types of birds can you think of? Robins, ostriches, hummingbirds, chickens. All of these are
birds, but they are also all very different from each other. There is an amazingly wide diversity of birds. Like
amphibians, reptiles, mammals, and fish, they are vertebrates. What does that mean? It means they have a backbone.
Birds have forelimbs modified as wings, but not all birds can fly.
Birds are in the class Aves. All birds have the following key features: they are endothermic (warm-blooded), have
two legs, and lay eggs.
Birds range in size from the tiny 2-inch bee hummingbird to the 9-foot ostrich (Figure 14.1 ). With approximately
10,000 living species, birds are the most numerous vertebrates with four limbs. They live in diverse habitats around
the globe, from the Arctic to the Antarctic.
The ostrich can reach a height of 9 feet
Pictured here are ostriches with young
in Namibia Africa.
The digestive system of birds is unique, with a gizzard that contains swallowed stones for grinding food. Birds do
not have teeth. What do you think the stones do? They help them digest their food. Defining characteristics of
modern birds also include:
High metabolism.
A four-chambered heart.
A beak with no teeth.
A lightweight but strong skeleton.
Production of hard-shelled eggs.
Which of the above traits do you think might be of importance to flight?
Adaptations for Flight
In comparing birds with other vertebrates, what do you think distinguishes them the most? In most birds, flight is
the most obvious difference. Birds have adapted their body plan for flight:
• Their skeleton is especially lightweight, with large air-filled spaces connecting to their respiratory system.
• Their neck bones are flexible. Birds that fly have a bony ridge along the breastbone that the flight muscles
attach to (Figure 14.2 ). This allows them to remain stable in the air as they fly.
• Birds also have wings that function as an aerofoil. The surface of the aerofoil is curved to help the bird control
and use the air currents to fly. Aerofoils are also found on planes.
What other traits do you think might be important for flight? Feathers help because they more lightweight than scales
or fur. A bird’s wing shape and size will determine how a species flies. For example, many birds have powered,
flapping flight at certain times, while at other times they soar, using up less energy (Figure 14.3 ).
14.1. BIRDS
A bony ridge along the breastbone blue
allows birds to remain stable as they fly.
One bird&#8217 s flight as seen in a
tern species.
About 60 living bird species are flightless, such as penguins, as were many extinct birds. Flightlessness often evolves
when birds live on isolated islands, probably due to limited resources and the absence of land predators. For example,
the flightless cormorant can no longer fly, but its wings are now adapted to swim in the sea (Figure 14.4 ).
A flightless cormorant can no longer fly
but uses its wings for swimming.
Reproduction in Birds
How do birds reproduce? We know that chickens lay eggs. But how do they do that?
It all starts with behavior aimed at attracting a mate. In birds, this will involve a type of display, usually performed
by the male. If successful, it will lead to breeding. Most male birds sing a type of song to attract females. Some
displays are very elaborate and may include dancing, aerial flights, or wing or tail drumming.
Listen here;feature=related for singing birds (4:53).
Click image to the left for more content.
Birds reproduce by internal fertilization. Like reptiles, birds have cloaca, or a single exit and entrance for sperm,
eggs, and waste. The male brings his sperm to the female cloaca. The sperm fertilizes the egg. The hard-shelled
eggs have a fluid-filled amnion, a thin membrane forming a closed sac around the embryo. Eggs are usually laid in
a nest.
14.1. BIRDS
How Do Birds Protect Their Offspring?
Why do you think eggs come in so many different colors? If an egg is hidden in a hole or burrow, away from
predators, then the eggs are most often pale or white.
Birds that make nests in the open have camouflaged eggs (Figure 14.5 ). This gives the eggs protection against
predation. Some species, like ground-nesting nightjars, have pale eggs, but the birds camouflage the eggs with their
feathers. To protect their young, different species of birds make different nests. Many are elaborate, shaped like
cups, domes, plates, mounds or burrows. The albatross, however, makes a nest that is simply a scrape on the ground.
Still others, like the common guillemot, do not use nests. Instead, they lay their eggs on bare cliffs. Male emperor
penguins do not have a nest at all: they sit on eggs to keep them warm before they hatch, a process called incubation.
How else might a bird help protect its young from predators? Most species locate their nests in areas that are hidden,
in order to avoid predators. Large birds, or those that nest in groups, may build nests in the open, since they are more
capable of defending their young.
Nest and eggs of the common moorhen
showing camouflaged eggs.
Young Birds and Parental Care
In birds, 95% of species are monogamous, meaning the male and female remain together for breeding for a few
years or until one mate dies. Usually, the parents take turns incubating the eggs. Birds usually incubate their eggs
after the last one has been laid. In polygamous species, where there is more than one mate, one parent does all of
the incubating.
The length and type of parental care varies widely amongst different species of birds. At one extreme, in a group
of birds called the magapodes, parental care ends at hatching. In this case, the newly-hatched chick digs itself out
of the nest mound without parental help and can take care of itself right away. These birds are called precocial.
At the other extreme, many seabirds care for their young for extended periods of time, the longest being that of the
great frigatebird. Their chicks receive parental care for six months, or until they are ready to fly, and then take an
additional 14 months of being fed by the parents (Figure 14.6 ). These birds are the opposite of precocial birds, and
are called altricial.
In most animals, male parental care is rare. But it is very common in birds. Often tasks are shared between parents,
like defense of territory and nest site, incubation, and the feeding of chicks. Since birds often take great care of their
young, some birds have evolved a behavior called "brood parasitism." This happens when a bird leaves her eggs in
another bird’s nest. The host bird often accepts and raises the parasite bird’s eggs.
Some chicks, like those of the ancient murrelet, follow their parents out to sea the night after they hatch, in order to
avoid land predators. In most species, however, the young do not leave the nest until they can fly.
Great frigatebird adults are known to
care for their young for up to 20 months
after hatching the longest in a bird
species. Here a young bird is begging
for food.
Diversity of Birds
About 10,000 bird species belong to 29 different orders within the class Aves. They live and breed on all seven
continents. The tropics are home to the greatest biodiversity of birds.
Birds that live in different environments will encounter different foods and different predators. The feeding habits
of birds are related to the beak shape and size, as well as the shape of the feet. Birds can be carnivores, insectivores,
or generalists, feeding on a variety of foods. Some, such as hummingbirds, feed on nectar. Can you think of some
examples of beak shape and size that are adapted to the type of food a bird eats?
As mentioned above, the size and shape of the beak is related to the food the bird eats, and can vary greatly among
different birds. Parrots have down-curved, hooked bills, which are well-adapted for cracking seeds and nuts (Figure
14.7 ). Hummingbirds, on the other hand, have long, thin, pointed bills, which are adapted for getting the nectar out
of flowers (Figure 14.8 ).
Bird feet can also vary greatly among different birds. Some birds, like gulls and terns, have webbed feet used for
swimming or floating (Figure 14.9 ). Other birds, such as herons, gallinules, and rails, have four long spreading
14.1. BIRDS
The down-curved hooked bill of a scarlet macaw a large colorful parrot.
A long thin and pointed bill of the
Swallow-tailed Hummingbird.
toes, which are adapted for walking delicately in the wetlands (Figure 14.10 ). Now you can predict how the beaks
and feet of birds will look depending on where they live and the type of food they eat.
The webbed feet of a great blackbacked gull.
FIGURE 14.10
The long spreading toes of an American
purple gallinule.
Why Birds are Important
We are probably most familiar with birds as food. Around the world, people consume chicken, turkey, and even
more exotic birds, like ostriches. Can you think of other ways that birds are important?
a. In agriculture, humans harvest bird droppings for use as fertilizer.
14.1. BIRDS
b. Chickens are also used as an early warning system of human diseases, such as West Nile virus. Mosquitoes
carry the West Nile virus, bite young chickens and other birds, and infect them with the virus. When chickens
or other birds become infected, this warns humans that they may also become infected in the near future.
c. Nectar-feeding birds are important pollinators, meaning they move the pollen from flower to flower to help
fertilize the sex cells and create new plants. Many fruit-eating birds help disperse seeds.
d. Birds are often important to island ecology. In New Zealand, the Kereru and Kokako are important browsers,
or animals that eat or nibble on leaves, tender young shoots, or other vegetation (Figure 14.11 and Figure
14.12 ). Seabirds add nutrients to soil and to water with their production of guano.
e. Birds have important cultural relationships with humans. Birds are common pets in the Western world. Sometimes, people act cooperatively with birds. For example, the Borana people in Africa use birds to guide them
to honey that they use in food.
f. Birds also play prominent and diverse roles in folklore, religion, and popular culture, and have been featured
in art since prehistoric times, as in early cave paintings.
FIGURE 14.11
The kereru is an important browser
species in New Zealand.
Lesson Summary
• Birds are warm-blooded.
• Adaptations for flight involve features that are lightweight, flexible, strong and that take advantage of air
• Reproduction usually involves a courtship display, nest production, egg laying, incubation and parental care.
• With 10,000 bird species, there is a lot of diversity. Specialized structures are adapted for specific habitats or
living requirements.
• Birds are important economically, ecologically, and in human culture.
FIGURE 14.12
The kokako another important browser
species of New Zealand.
Review Questions
1. List three traits which are important for flight.
2. Give an example of how a bird’s breeding system is adapted to avoid predators.
Apply Concepts
3. Explain how the absence of land predators on islands results in flightlessness in birds.
4. A large bird that nests with other birds has pale eggs even though the environment is brown and the eggs stand
out to predators. Why have these birds not evolved camouflaged eggs?
Critical Thinking
5. You detect the presence of antibodies to the West Nile Virus in young chickens. How did the chickens get the
virus? What does this mean about human West Nile infections?
14.1. BIRDS
Further Reading / Supplemental Links
Department of Health, Florida. Available on the web at:
Oliver L. Austin, Birds of the World. Western Publishing Company, Inc., New York, 1961.
Points to Consider
Mammals are next.
• Birds and mammals are the only warm-blooded vertebrates. As in birds, mammals exhibit wide diversity and
live in varied habitats. Based on what you know about adaptations in birds, how do you think mammalian
limbs are adapted for movement in different habitats?
• As in birds, mammals have different foods they eat depending on their environment. Instead of beaks, mammals have different kinds of teeth. In what way(s) do you think teeth in mammals are adapted for different
kinds of diets?
Lesson Objectives
List and describe general traits of mammals.
Compare reproduction in monotremes, marsupials and placental mammals.
Describe how mammals can be grouped according to their anatomy and their habitats.
Explain how non-human mammals can benefit people and how they play an ecological role.
Check Your Understanding
• Mammals are warm-blooded, like birds. What traits do you think they have in common because of this?
• Describe courtship displays in birds. As you learn about mammals, think about how their courtship is similar
to or differs from that seen in birds.
harem A group of females followed or accompanied by a fertile male; this male excludes other males access to
the group.
mammary glands Specialized sweat glands that produce milk.
marsupial A type of mammal where the female has an abdominal pouch or skin fold within which are mammary
glands and a place for raising the young.
monotremes A group of mammals that lays eggs and feeds their young by “sweating” milk from patches on their
neocortex Site of the cerebral cortex where most of higher brain functions occur.
placental A type of mammal that has a placenta that nourishes the fetus and removes waste products.
sexual dimorphism Extreme difference between the sexes.
Characteristics of Mammals
What is a mammal? These animals range from bats, cats and rats to dogs, monkeys and whales. They walk, run,
swim and fly. They live in the ocean, fly in the sky, walk on the prairies, and run in the savanna.
What allows them to live in such diverse environments? They have evolved specialized traits, unlike any other
group of animal. There is a tremendous amount of diversity within the group in terms of reproduction, habitat,
and adaptation for living in those different habitats. Some of their traits directly benefit people, while also playing
important ecological roles.
Mammals (class Mammalia) are endothermic (warm-blooded) vertebrate animals with a number of unique characteristics. In most mammals, these include:
The presence of hair or fur.
Sweat glands.
Glands specialized to produce milk, known as mammary glands.
Three middle ear bones.
A neocortex region in the brain, which specializes in seeing and hearing.
Specialized teeth.
A four-chambered heart.
All mammals, except for one, are viviparous, meaning they produce live young instead of laying eggs. The
monotremes, however, have birdlike and reptilian characteristics, such as laying eggs and a cloaca.
There are approximately 5,400 mammalian species, ranging in size from the tiny 1-2 inch bumblebee bat to the
108-foot blue whale. These are distributed in about 1,200 genera, 153 families and 29 orders.
Mammals are also the only animal group that evolved to live on land and then back to live in the ocean! Whales,
dolphins and porpoises have all adapted from land-dwelling creatures to a life of swimming and reproducing in the
water (Figure 14.13 ). Whales have evolved into the largest mammals.
FIGURE 14.13
Dolphins have all adapted to swimming
and reproducing in water.
Reproduction in Mammals
There are similarities between the reproductive habits or mammals and those of reptiles, birds, and amphibians. See
if you can spot them.
The egg-laying monotremes, such as echidnas (Figure 14.14 ) and platypuses (Figure 14.15 ), use one opening, the
cloaca, to urinate, release waste, and reproduce, just as lizards and birds do. They lay leathery eggs, similar to those
of lizards, turtles and crocodilians. Monotremes feed their young by “sweating” milk from patches on their bellies,
since they lack the nipples present on other mammals.
FIGURE 14.14
The echidna is a member of the
monotremes the most primitive order of
FIGURE 14.15
Another monotreme the platypus like
other mammals in this order lays eggs
and has a single opening for the urinary
genital and digestive organs.
All other mammals give birth to live young and belong to one of two different categories:
a. Marsupial: most female marsupials have an abdominal pouch or skin fold where there are mammary glands
and a place for raising the young (Figure 14.16 ).
b. Placental: female placentals have a placenta that feeds the fetus and removes waste products.
Some mammals are alone until a female can become pregnant. Others form social groups with big differences
between sexes, a trait called sexual dimorphism. Dominant males are those that are largest or best-armed. These
males usually have an advantage in mating. They may also keep other males from mating with females within a
group. This is seen in elephant seals (Figure 14.17 ). This group of females is called a harem.
FIGURE 14.16
A marsupial mammal this Eastern grey
kangaroo has a joey young kangaroo in
its abdominal pouch.
FIGURE 14.17
A mating system with a harem of many
females and one male as seen in
the seal species. Think back to what
you learned about courtship displays in
birds. How is such conduct in mammals
similar or different
Groups of Mammals
As is true for most animal groups, mammal groups can be characterized a number of ways: according to their
anatomy, the habitats where they live, or their feeding habits.
Most mammals belong to the placental group. Within this group are several subgroups including:
Lagomorphs, such as hares and rabbits.
Rodents, including rats, mice and other small, gnawing mammals.
Carnivores, such as cats, dogs, bears and other meat eaters (Figure 14.18 ).
Insectivores, including moles and shrews (Figure 14.19 ).
Bats and primates.
Ungulates, including hoofed animals such deer, sheep, goats, pigs, buffalo and elephants, as well as marine
mammals, such as whales and manatees (Figure 14.20 ).
FIGURE 14.18
A caracal hunting in the Serengeti.
FIGURE 14.19
One of the subgroups of placental mammals is the insectivores including moles
and shrews. Pictured here is the Northern short-tailed shrew.
Why do you think the above groups of animals are placed together? Can you think of some examples of tooth type
that are adapted for a mammal’s diet? Or types of limbs that are adapted for living in different types of habitats?
FIGURE 14.20
The ungulates hoo f ed animals like the
giraffe here is one of the subgroups belonging to the placental mammals.
Mammals can also be grouped according to the adaptations they form to live in a certain habitat.
For example, terrestrial mammals with leaping kinds of movement, as in some marsupials and in lagomorphs,
typically live in open habitats. Other terrestrial mammals are adapted for running, such as dogs or horses.
Still others, such as elephants, hippopotamuses, and rhinoceroses, move slowly.
Other mammals are adapted for living in trees, such as many monkeys (Figure 14.21 ). Others live in water, such
as manatees, whales, dolphins, and seals. Still others are adapted for flight, like bats, or for gliding, like some
marsupials and rodents.
FIGURE 14.21
This howler monkey shows adaptations
for an arboreal existence.
Importance of Mammals
Mammals are significant in the ways they benefit people, the economy, and ecosystems. Ecologically, nectar-feeding
and fruit-eating bats (Figure 14.22 ) play an important role in plant pollination and seed dispersal, respectively.
FIGURE 14.22
Bats like this Egyptian fruit bat belong
to another subgroup of placental mammals. Ecologically fruit bats play an important role in seed dispersal.
Importance to Humans
We see examples of mammals (other than people!) serving our needs everywhere. We have pets that are mammals,
most commonly dogs and cats. Mammals are also used for transport (horses, donkeys, mules, and camels), food
(cows and goats), and work (dogs, horses, and elephants). See a working dog in Figure 14.23 .
FIGURE 14.23
A Labrador retriever working as an assistance dog.
Mammals’ more highly developed brains have made them ideal for use by scientists in studying such things as
learning, as seen in maze studies of mice and rats.
Cultural Importance
Mammals have also played a significant role in different cultures’ folklore and religion. For example, the grace and
power of the cougar have been admired in the cultures of the native peoples of the Americas. The Inca city of Cuzco
is designed in the shape of a cougar, and the thunder god of the Inca, Viracocha, has been associated with the animal.
In North America, mythological descriptions of the cougar have appeared in stories of a number of Native American
Lesson Summary
• Organisms in the class Mammalia are distinguished by the presence of hair, sweat glands, three middle ear
bones and a neocortex area in the brain.
• There is a lot of variation in mammalian reproductive systems. Mammals consist of both the egg-laying
monotremes and those that are viviparous. The latter group includes marsupial and placental mammals.
• The 5,400 species of mammals can be grouped according to physical features as well as the type of habitat.
• Mammals have specific adaptations for living on land, in trees, in water and for flight.
• Non-primate mammals have an form important relationships with humans and play key ecological roles.
Review Questions
1. What are three main characteristics of mammals?
2. What are two ways that monotremes are different from viviparous mammals?
Apply Concepts
3. With respect to characteristics of feet, limbs and tails, what features would you expect mammals to have for:
• Jumping?
• Living in trees?
4. Give examples of two different adaptations of limbs in mammals, naming a mammal species, a structure, and how
it is adapted.
Critical Thinking
5. Instead of beaks, as in birds, mammals have different kinds of teeth. Incisors are specialized for cutting and
nipping, premolars for shearing and grinding, and canines for piercing. Based on what you know about mammal
diets, name two mammal species, the kind of diet they eat, and one type of specialized teeth that would be best
adapted for the diet.
Further Reading / Supplemental Links
Unabridged Dictionary, Second Edition. Random House, New York, 1998.
Points to Consider
Next we turn to primates.
• Think of some significant similarities between the mammals you read about in this lesson with those in the
next lesson, particularly human beings.
• What do you think are some of the significant adaptations in the evolution of primates?
Primates and Humans
Lesson Objectives
List and describe general traits of primates.
Summarize mating systems of primates.
Review the types of habitats primates can be found in.
Describe the three main groupings of primates.
List the traits of the hominids, their diet, reproduction and social system.
Check Your Understanding
• What are general traits of mammals?
• Describe the mating systems in mammals.
hybrid The offspring of different species, genera, varieties or breeds.
omnivorous Eating both plant and animal material.
pentadactyl Having five fingers or toes.
quadrupedal four-footed
What are Primates?
If primates are mammals, what makes them seem so different? Primates, including humans, have several unique
features only belonging to this group of mammals. Some adaptations give primates advantages that allow them to
live in certain habitats, such as in trees. Other features have allowed them to adapt to complex social and cultural
The biological order primates are mostly omnivorous, meaning they eat both plant and animal material. The order
contains all of the species commonly related to the lemurs (Figure 14.24 ), monkeys (Figure 14.25 ) and apes
(Figure 14.26 ). The order of primates includes humans (Figure ?? ).
Key features of primates include:
• Five fingers, known as pentadactyl.
FIGURE 14.24
A ring tailed lemur and twins. Lemurs
belong to the prosimian group of primates.
FIGURE 14.25
One of the New World monkeys a squirrel monkey.
FIGURE 14.26
Chimpanzees pictured here belong to
the great apes one of the groups of primates.
• Similar teeth.
• A nonspecialized body plan,
• Certain eye orbit characteristics, such as a postorbital bar, or a bone that runs around the eye socket.
An opposable thumb is a finger that allows a grip that can hold objects. While this opposable thumb is a characteristic feature of this group, other orders, such as opossums, also have this feature.
Big Brains
In intelligent mammals, such as primates, the cerebrum is larger compared to the rest of the brain. A larger cerebrum
allows primates to develop higher level of intelligence. Primates have the ability to learn new behaviors. They also
engage in complex social interactions, such as fighting and play.
Social Relationships
Old World species, such as apes and some monkeys as seen in Figure 14.26 and Figure 14.27 ), tend to have
significant size differences between the sexes. This is known as sexual dimorphism. Males tend to be slightly more
than twice as heavy as females. This dimorphism may have evolved when one male had to defend many females.
New World species, including tamarins (Figure 14.28 ) and marmosets (Figure 14.29 ), form pair bonds, which is a
partnership between a mating pair that lasts at least one season. The pair cooperatively raise the young and generally
do not show significant size difference between the sexes.
FIGURE 14.27
An Old World monkey a species of
macaque in Malaysia.
FIGURE 14.28
A New World species of monkey a
FIGURE 14.29
Another New World species of monkey
the common marmoset.
Where do Non-human Primates Live?
Non-human primates live mostly in Central and South America, Africa and South Asia. Since primates evolved
from arboreal (living in trees) animals, many modern species live mostly in trees.
Other species live on land most of the time, such as baboons (Figure 14.30 ) and the Patas monkey. Only a few
species live on land all of the time, for example, the gelada and humans.
Primates live in a diverse number of forested habitats, including rain forests, mangrove forests and mountain forests
to altitudes of over 9,800 feet. The combination of opposable thumbs, short fingernails and long, inward-closing
fingers has allowed some species to develop brachiation, or the ability to move by swinging their arms from one
branch to another (Figure 14.31 ). Another feature for climbing are expanded finger-like parts, such as those in
tarsiers, which improve grasping (Figure 14.32 ).
A few species, such as the proboscis monkey, De Brazza’s monkey and Allen’s swamp monkey evolved webbed
fingers so they can swim and live in swamps and aquatic habitats. Some species, such as the rhesus macaque and the
Hanuman langur, can even live in cities by eating human garbage.
FIGURE 14.30
Baboons are partially terrestrial. Pictured here is a mother baboon and her
young in Tanzania.
Primate Classification
The primate order is divided informally into three main groupings:
a. Prosimians: The prosimians constitute species whose bodies most closely resemble that of the early protoprimates, the earliest examples of primates (Figure 14.33 ). Prosimians include the lemurs.
b. New World monkeys: The New World monkeys include the capuchin, howler and squirrel monkeys, who live
exclusively in the Americas.
c. Old World monkeys and apes: The Old World monkeys and the apes (all except for humans, who inhabit the
entire earth) inhabit Africa and southern and central Asia.
FIGURE 14.31
A gibbon shows how its limbs are modified for hanging from trees.
FIGURE 14.32
A species of tarsier with expanded digits used for grasping branches.
FIGURE 14.33
One of the prosimians a greater bush
baby Kenya.
The Human Family
The great apes are the members of the biological family Hominidae, which includes four living genus: chimpanzees,
gorillas, humans, and orangutans.
Hominids are large, tailless primates, ranging in size from the pygmy chimpanzee, at 66-88 pounds in weight, to the
gorilla, at 300-400 pounds (Figure 14.34 ). In all species, the males are, on average, larger and stronger than the
Most living primate species are four-footed, but all are able to use their hands for gathering food or nesting materials.
In some cases, hands are used as tools, such as when gorillas use sticks to measure the depth of water. Chimpanzees
sharpen sticks to use as spears in hunting; they also use sticks to gather food and to “fish” for termites (Figure 14.35
Most primate species eat both plants and meat (omnivorous), but fruit is the preferred food among all but humans.
In contrast, humans eat a large amount of highly processed, low fiber foods, and unusual proportions of grains and
vertebrate meat.
Human teeth and jaws are markedly smaller for our size than those of other apes. Humans may have been eating
cooked food for a million years or more, so perhaps their teeth adapted to eating cooked food.
Gestation (pregnancy) lasts 8-9 months and usually results in the birth of a single offspring. The young are born
helpless, and thus they need parental care for long periods of time.
Compared with most other mammals, great apes have a long adolescence and are not fully mature until 8-13 years
of age (longer in humans). Females usually give birth only once every few years.
Gorillas and chimpanzees live in family groups of approximately five to ten individuals, although larger groups are
sometimes observed. The groups include at least one dominant male, and females leave the group when they can
mate. Orangutans, however, generally live alone.
Genetic and Behavioral Similarities
Gorillas, chimpanzees and humans are classified together in the subfamily, the Hominidae, because they have more
than 97% of their DNA in common!
All organisms in the Hominidae also communicate with some kind of language or create simple cultures beyond the
family or band (a group of animals functioning together).
Can you think of human characteristics that are unique to humans? Some say that only humans have the capacity for
empathy, or the ability to understand and share the feelings of others. Do you think there are other characteristics
that only humans have?
FIGURE 14.34
A gorilla mother and baby members of
the great apes at Volcans National Park
Rwanda. The gorilla is the largest of the
hominids weighing up to 309-397 lbs.
FIGURE 14.35
Tool using in a primate. A chimpanzee
uses a stick to &#8220 fish&#8221 for
termites and then pictured here extracts the insects.
Lesson Summary
• Primates are characterized by pentadactyly, similar teeth and certain eye orbit features. Primates also have
opposable thumbs and a large cerebrum relative to the rest of the brain.
• Old World species tend to have significant sexual dimorphism, whereas New World species generally do not
show significant sexual differences.
• Many primates live in a variety of forested habitats, whereas others are partially terrestrial, and some, like the
gelada and humans, are fully terrestrial. A few species are adapted for living in aquatic habitats.
• There are three subgroups within the primates order: prosimians, including the lemurs; New World monkeys,
and the Old World monkeys and the apes.
• The great apes, consisting of seven species, are large, tailless primates, with sexual dimorphism. Most species
are quadrupedal, but all are able to use their hands.
• Most great apes are omnivorous, but fruit is the preferred food among all species but humans.
• The great apes have unique reproductive and parental care features, especially when compared with most other
• Gorillas, chimpanzees and humans share some common characteristics.
Review Questions
1. Name three characteristics of Primates?
2. What theory might explain why human teeth and jaws are smaller for our size than those of other apes?
Apply Concepts
3. All primates have opposable thumbs. List two ways in which non-human primates might use opposable thumbs.
4. Primates are thought to have developed several of their traits and habits while living in trees. What primate
features might be an advantage in an arboreal (tree) habitat?
Critical Thinking
5. Gorillas and chimpanzees live in family groups of around five to 10 individuals. What might be two of their
possible strategies for feeding, when fruit is hard to find?
Further Reading / Supplemental Links
Points to Consider
The behavior of animals is next.
• What do you think the "behavior" of animals refers to? Give some examples.
MS Behavior of Animals
You have probably seen a spider web before. You may even know that spiders create webs to catch their prey. But
have you thought about how the spider learns to spin a web? Are young spiders taught by older spiders? Or do
spiders just "know" how to spin a web when they are born?
Animals have different behaviors for different reasons. Some they learn. Some they are born knowing how to do. In
the case of spiders, they know from birth how to spin a web. No one teaches them.
Is this different from humans? Are there things you just know how to do? Or do you have to learn every new
behavior? Find the answers to these questions as you read.
Understanding Animal Behavior
Lesson Objectives
Give examples of animal behavior.
Explain why animal behavior is important.
Describe innate behavior and how it evolves.
List ways that behavior can be learned.
Check Your Understanding
• What is an animal?
• What are some examples of animals that behave very differently from each other?
animal behavior Any way that animals act, either alone or with other animals.
conditioning Way of learning that involves a reward or punishment.
habituation Learning to get used to something that is not dangerous, after being exposed to it for awhile.
innate behavior Any behavior that occurs naturally in all animals of a given species.
insight learning Learning from past experiences and reasoning.
instinct Any behavior that occurs naturally in all animals of a given species; another term for an innate behavior.
learned behavior Behavior that occurs only after experience or practice.
observational learning Learning by watching and copying the behavior of someone else.
reflex behaviors The only truly innate behaviors in humans, occurring mainly in babies.
Examples of Animal Behavior
Barking, purring, and playing are just some of the ways that dogs and cats behave. These are examples of animal
Animal behavior is any way that animals act, either alone or with other animals. Can you think of other examples
of animal behavior? What about insects and birds? How do they behave? The pictures in Figure 15.1 , Figure 15.2
, Figure 15.3 , Figure 15.4 , Figure 15.5 , Figure 15.6 , and Figure 15.7 show just some of the ways that these and
other animals act. Look at the pictures and read about the behaviors. Think about why the animal is behaving that
This cat is stalking a mouse. It is a
hunter by nature.
This spider is busy spinning a web. If
you have ever walked into a spider web
you know how sticky a spider web can
be. Why do spiders spin webs
This mother dog is nursing her puppies.
In what other ways do mother dogs care
for their puppies
This bird is using its beak to add more
grass to its nest. What will the bird use
its nest for
This wasp is starting to build a nest.
Have you seen nests like this on buildings where you live Why do wasps build
This rabbit is running away from a fox.
Did you ever see a rabbit run Do you
think you could run that fast
This lizard is perched on a rock in
the sun. Lizards like to lie on rocks
and &#8220 sun&#8221
Do you know why
Importance of Animal Behavior
Why do animals behave the way they do? The answer to this question depends on what the behavior is. A cat chases
a mouse to catch it. A spider spins its sticky web to trap insects. A mother dog nurses her puppies to feed them.
All of these behaviors have the same purpose: getting or providing food. All animals need food for energy. They
need energy to move around. In fact, they need energy just to stay alive. Baby animals also need energy to grow and
Birds and wasps build nests to have a safe place to store their eggs and raise their young. Many other animals build
nests for the same reason. Animals protect their young in other ways, as well. For example, a mother dog not only
nurses her puppies. She also washes them with her tongue and protects them from strange people or other animals.
All of these behaviors help the young survive and grow up to be adults.
Rabbits run away from foxes and other predators to stay alive. Their speed is their best defense. Lizards sun
themselves on rocks to get warm because they cannot produce their own body heat. When they are warmer, they can
move faster and be more alert. This helps them escape from predators, as well as find food.
All of these animal behaviors are important. They help the animals get food for energy, make sure their young
survive, or ensure that they survive themselves. Behaviors that help animals or their young survive increase the
animals’ fitness. You read about fitness in the Evolution chapter. Animals with higher fitness have a better chance
of passing their genes to the next generation. If genes control behaviors that increase fitness, the behaviors become
more common in the species. This is called evolution by natural selection.
Innate Behavior
All of the behaviors shown in the images above are ways that animals act naturally. They don’t have to learn how
to behave in these ways. Cats are natural-born hunters. They don’t need to learn how to hunt. Spiders spin their
complex webs without learning how to do it from other spiders. Birds and wasps know how to build nests without
being taught. These behaviors are called innate.
An innate behavior is any behavior that occurs naturally in all animals of a given species. An innate behavior is also
called an instinct. The first time an animal performs an innate behavior, the animal does it well. The animal does
not have to practice the behavior in order to get it right or become better at it. Innate behaviors are also predictable.
All members of a species perform an innate behavior in the same way. From the examples described above, you can
probably tell that innate behaviors usually involve important actions, like eating and caring for the young.
There are many other examples of innate behaviors. For example, did you know that honeybees dance? The honeybee in Figure 15.8 has found a source of food. When the bee returns to its hive, it will do a dance, called the waggle
dance. The way the bee moves during its dance tells other bees in the hive where to find the food. Honeybees can
do the waggle dance without learning it from other bees, so it is an innate behavior.
Besides building nests, birds have other innate behaviors. One example occurs in gulls. A mother gull and two of
her chicks are shown in Figure 15.9 . One of the chicks is pecking at a red spot on the mother’s beak. This innate
behavior causes the mother to feed the chick. In many other species of birds, the chicks open their mouths wide
whenever the mother returns to the nest. This is what the baby birds in Figure 15.10 are doing. This innate behavior,
called gaping, causes the mother to feed them.
Another example of innate behavior in birds is egg rolling. It happens in some species of water birds, like the graylag
goose shown in Figure 15.11 . Graylag geese make nests on the ground. If an egg rolls out of the nest, a mother
goose uses her bill to push it back into the nest. Returning the egg to the nest helps ensure that the egg will hatch.
When this honeybee goes back to its
hive it will do a dance to tell the other
bees in the hive where it found food.
This mother gull will feed her chick after it pecks at a red spot on her beak.
Both pecking and feeding behaviors are
FIGURE 15.10
When these baby birds open their
mouths wide the mother instinctively
feeds them. This innate behavior is
called gaping.
FIGURE 15.11
This female graylag goose is a groundnesting water bird. Before her chicks
hatch the mother protects the eggs.
She will use her bill to push eggs back
into the nest if they roll out. This is
an example of an innate behavior. How
could this behavior increase the mother
goose&#8217 s fitness
Innate Behavior in Human Beings
All animals have innate behaviors, even human beings. Can you think of human behaviors that do not have to be
learned? Chances are, you will have a hard time thinking of any. The only truly innate behaviors in humans are
called reflex behaviors. They occur mainly in babies. Like innate behaviors in other animals, reflex behaviors in
human babies may help them survive.
An example of a reflex behavior in babies is the sucking reflex. Newborns instinctively suck on a nipple that is
placed in their mouth. It is easy to see how this behavior evolved. It increases the chances of a baby feeding and
surviving. Another example of a reflex behavior in babies is the grasp reflex. This behavior is shown in Figure 15.12
. Babies instinctively grasp an object placed in the palm of their hand. Their grip may be surprisingly strong. How
do you think this behavior might increase a baby’s chances of surviving?
FIGURE 15.12
One of the few innate behaviors in human beings is the grasp reflex. It occurs
only in babies.
Learned Behavior
Just about all other human behaviors are learned. Learned behavior is behavior that occurs only after experience
or practice. Learned behavior has an advantage over innate behavior. It is more flexible. Learned behavior can be
changed if conditions change. For example, you probably know the route from your house to your school. Assume
that you moved to a new house in a different place, so you had to take a different route to school. What if following
the old route was an innate behavior? You would not be able to adapt. Fortunately, it is a learned behavior. You can
learn the new route just as you learned the old one.
Although most animals can learn, animals with greater intelligence are better at learning and have more learned
behaviors. Humans are the most intelligent animals. They depend on learned behaviors more than any other species.
Other highly intelligent species include apes, our closest relatives in the animal kingdom. They include chimpanzees
and gorillas. Both are also very good at learning behaviors.
You may have heard of a gorilla named Koko. The psychologist Dr. Francine Patterson raised Koko. Dr. Patterson
wanted to find out if gorillas could learn human language. Starting when Koko was just one year old, Dr. Patterson
taught her to use sign language. Koko learned to use and understand more than 1,000 signs. Koko showed how
much gorillas can learn. See A Conversation with Koko at for additional
Think about some of the behaviors you have learned. They might include riding a bicycle, using a computer, and
playing a musical instrument or sport. You probably did not learn all of these behaviors in the same way. Perhaps
you learned some behaviors on your own, just by practicing. Other behaviors you may have learned from other
people. Humans and other animals can learn behaviors in several different ways.
The following methods of learning will be explored below:
Habituation (forming a habit).
Observational learning.
Insight learning.
Habituation is learning to get used to something after being exposed to it for a while. Habituation usually involves
getting used to something that is annoying or frightening, but not dangerous. Habituation is one of the simplest ways
of learning. It occurs in just about every species of animal.
You have probably learned through habituation many times. For example, maybe you were reading a book when
someone turned on a television in the same room. At first, the sound of the television may have been annoying. After
awhile, you may no longer have noticed it. If so, you had become habituated to the sound.
Another example of habituation is shown in Figure 15.13 . Crows and most other birds are usually afraid of people.
They avoid coming close to people, or they fly away when people come near them. The crows landing on this
scarecrow have gotten used to a “human” in this place. They have learned that the scarecrow poses no danger. They
are no longer afraid to come close. They have become habituated to the scarecrow.
Can you see why habituation is useful? It lets animals ignore things that will not harm them. Without habituation,
animals might waste time and energy trying to escape from things that are not really dangerous.
Observational Learning
Observational learning is learning by watching and copying the behavior of someone else. Human children learn
many behaviors this way. When you were a young child, you may have learned how to tie your shoes by watching
your dad tie his shoes. More recently, you may have learned how to dance by watching a pop star dancing on TV.
Most likely you have learned how to do math problems by watching your teachers do problems on the board at
school. Can you think of other behaviors you have learned by watching and copying other people?
Other animals also learn through observational learning. For example, young wolves learn to be better hunters by
watching and copying the skills of older wolves in their pack.
Another example of observational learning is how some monkeys have learned how to wash their food. They learned
by watching and copying the behavior of other monkeys.
FIGURE 15.13
This scarecrow is no longer scary to
these crows. They have become used
to its being in this spot and learned that
it is not dangerous. This is an example
of habituation.
Conditioning is a way of learning that involves a reward or punishment. Did you ever train a dog to fetch a ball
or stick by rewarding it with treats? If you did, you were using conditioning. Another example of conditioning
is shown in Figure 15.14 . This lab rat has been taught to “play basketball” by being rewarded with food pellets.
Conditioning also occurs in wild animals. For example, bees learn to find nectar in certain types of flowers because
they have found nectar in those flowers before.
Humans learn behaviors through conditioning, as well. A young child might learn to put away his toys by being
rewarded with a bedtime story. An older child might learn to study for tests in school by being rewarded with better
grades. Can you think of behaviors you learned by being rewarded for them?
Conditioning does not always involve a reward. It can involve a punishment instead. A toddler might be punished
with a time-out each time he grabs a toy from his baby brother. After several time-outs, he may learn to stop taking
his brother’s toys.
A dog might be scolded each time she jumps up on the sofa. After repeated scolding, she may learn to stay off the
sofa. A bird might become ill after eating a poisonous insect. The bird may learn from this “punishment” to avoid
eating the same kind of insect in the future.
Learning by Playing
Most young mammals, including humans, like to play. Play is one way they learn skills they will need as adults.
Think about how kittens play. They pounce on toys and chase each other. This helps them learn how to be better
predators when they are older. Big cats also play. The lion cubs in Figure 15.15 are playing and practicing their
hunting skills at the same time. The dogs in Figure 15.16 are playing tug-of-war with a toy. What do you think they
FIGURE 15.14
This rat has been taught to put the ball
through the hoop by being rewarded
with food for the behavior. This is an
example of conditioning. What do you
think would happen if the rat was no
longer rewarded for the behavior
are learning by playing together this way?
Other young animals play in different ways. For example, young deer play by running and kicking up their hooves.
This helps them learn how to escape from predators.
FIGURE 15.15
These two lion cubs are playing. They
are not only having fun. They are also
learning how to be better hunters.
Human children learn by playing as well. For example, playing games and sports can help them learn to follow rules
and work with others. The baby in Figure 15.17 is playing in the sand. She is learning about the world through play.
What do you think she might be learning?
FIGURE 15.16
They are really playing. This play fighting can help them learn how to be better
FIGURE 15.17
Playing in a sandbox is fun for young
children. It can also help them learn
about the world. For example this child
may be learning that sand is soft.
Insight Learning
Insight learning is learning from past experiences and reasoning. It usually involves coming up with new ways
to solve problems. Insight learning generally happens quickly. An animal has a sudden flash of insight. Insight
learning requires relatively great intelligence. Human beings use insight learning more than any other species. They
have used their intelligence to solve problems ranging from inventing the wheel to flying rockets into space.
Think about problems you have solved. Maybe you figured out how to solve a new type of math problem or how to
get to the next level of a video game. If you relied on your past experiences and reasoning to do it, then you were
using insight learning.
One type of insight learning is making tools to solve problems. Scientists used to think that humans were the only
animals intelligent enough to make tools. In fact, tool-making was believed to set humans apart from all other
In 1960, primate expert Jane Goodall discovered that chimpanzees also make tools. She saw a chimpanzee strip
leaves from a twig. Then he poked the twig into a hole in a termite mound. After termites climbed onto the twig,
he pulled the twig out of the hole and ate the insects clinging to it. The chimpanzee had made a tool to “fish” for
termites. He had used insight to solve a problem. Since then, chimpanzees have been seen making several different
types of tools. For example, they sharpen sticks and use them as spears for hunting. They use stones as hammers to
crack open nuts.
Scientists have also observed other species of animals making tools to solve problems. A crow was seen bending a
piece of wire into a hook. Then the crow used the hook to pull food out of a tube.
An example of a gorilla using a walking stick is shown in Figure 15.18 . Behaviors such as these show that other
species of animals can use their experience and reasoning to solve problems. They can learn through insight.
Lesson Summary
Animal behavior is any way that animals act. This behavior may be either alone or with other animals.
Behaviors that increase fitness can evolve over time. This process occurs by natural selection.
Innate behavior is behavior that occurs naturally. This behavior occurs in all members of a species.
Learned behavior is behavior that is learned. It occurs only through experience or practice.
Review Questions
1. Give two examples of animal behavior.
2. Define innate behavior.
3. State three ways that behavior can be learned.
Apply Concepts
4. Identify one drawback of innate behavior.
5. What is the difference between learned behavior and innate behavior?
6. Why is play important for baby animals?
FIGURE 15.18
This gorilla is using a branch as a tool.
She is leaning on it to keep her balance
while she reaches down into swampy
water to catch a fish.
7. Explain how you could use conditioning to teach a dog to sit.
Critical Thinking
8. Explain how egg rolling by graylag geese is likely to have evolved.
9. Describe how the grasp reflex might help a baby survive.
10. A crow was seen dropping nuts on a rock to crack the shells and then eating the nut meats. No other crows in the
flock were ever observed cracking nuts in this way. What type of learning could explain the behavior of this crow?
Further Reading / Supplemental Links
CK-12 Foundation. High School Biology, Chapter 34, “Animal Behavior.”
• Melvin Berger. Dogs Bring Newspapers but Cats Bring Mice: and Other Fascinating Facts about Animal
Behavior. Scholastic, 2004.
• Paolo Casale and Gian Paolo Faescini. Animal Behavior: Instinct, Learning, Cooperation. Barrons Juveniles,
Pavlov’s Dogs Get Conditioned can be viewed at (3:10).
Click image to the left for more content.
Points to Consider
Next, we discuss the types of animal behaviors.
• Did you ever watch a long line of ants marching away from their ant hill? What were they doing? How were
they able to work together? What do you think explains group behaviors such as this?
Types of Animal Behavior
Lesson Objectives
List ways that animals communicate.
Describe social behavior in animals.
Explain the purpose of mating behavior.
Describe how animals defend their territory.
Identify animal behaviors that occur in cycles.
Check Your Understanding
• What is an animal?
• Give examples of a wide variety of animals.
• List some behaviors that animals, such as spiders and rabbits, have in common.
biological clock Tiny structure in the brain that controls circadian rhythms.
circadian rhythms An organism’s daily cycles of behavior.
communication Any way that animals share information.
cooperation Working together with others for the common good.
courtship behaviors Special behaviors that help attract a mate.
display behavior Fixed set of actions that carries a specific message.
hibernation State in which an animal’s body processes are slower than usual.
language Use of symbols or sounds to communicate.
mating Pairing of an adult male and female to produce young.
migration Movement of animals from one place to another; often seasonal.
social animals Animals that live in groups with other members of their species.
What does the word "communication" make you think of? Talking on a cell phone? Texting? Writing? Those are
just a few of the ways that human beings communicate. Most other animals also communicate. Communication is
any way that animals share information, and they do this in many different ways.
Do all animals talk to each other? Probably not, but many do communicate. Like human beings, many other animals
live together in groups. Some insects, including ants and bees, are well known for living in groups. In order for
animals to live together in groups, they must be able to communicate with each other.
Animal communication, like most other animal behaviors, increases the ability to survive and have offspring. This is
known as fitness. Communication increases fitness by helping animals find food, defend themselves from predators,
mate, and care for offspring.
Ways That Animals Communicate
Communication with Sound
Some animals communicate with sound. Most birds communicate this way. Birds use different calls to warn other
birds of danger, or to tell them to flock together. Many other animals also use sound to communicate. For example,
monkeys use warning cries to tell other monkeys in their troop that a predator is near. Frogs croak to attract female
frogs as mates. Gibbons use calls to tell other gibbons to stay away from their area.
Communication with Sight
Another way some animals communicate is with sight. By moving in certain ways, or by “making faces,” they show
other animals what they mean. Most primates communicate in this way. For example, a male chimpanzee may
raise his arms and stare at another male chimpanzee. This warns the other chimpanzee to keep his distance. The
chimpanzee in Figure 15.19 may look like he is smiling, but he is really showing fear. He is communicating to other
chimpanzees that he will not challenge them.
Look at the peacock in Figure 15.20 . Why is he raising his beautiful tail feathers? He is also communicating. He
is showing females of his species that he would be a good mate.
Communication with Scent
Some animals communicate with scent. They release chemicals that other animals of their species can smell or detect
in some other way. Ants release many different chemicals. Other ants detect the chemicals with their antennae. This
explains how ants are able to work together. The different chemicals that ants produce have different meanings.
Some of the chemicals signal all the ants in a group to come together. Other chemicals warn of danger. Still other
chemicals mark trails to food sources. When an ant finds food, it marks the trail back to the nest by leaving behind
a chemical on the ground. Other ants follow the chemical trail to the food.
Many other animals also use chemicals to communicate. You have probably seen male dogs raise their leg to urinate
on a fire hydrant or other object. Did you know that the dogs were communicating? They were marking their area
with a chemical in their urine. Other dogs can smell the chemical. The scent of the chemical tells other dogs to stay
FIGURE 15.19
This chimpanzee is communicating with
his face. His expression is called a
&#8220 fear grin.&#8221 It tells other
chimpanzees that he is not a threat.
FIGURE 15.20
This peacock is using his tail feathers to communicate. What is he
&#8220 saying&#8221
Human Communication
Like other animals, humans communicate with one another. They mainly use sound and sight to share information.
The most important way that humans communicate is with language. Language is the use of symbols to communicate. In human languages, the symbols are words. They stand for many different things. Words stand for things,
people, actions, feelings, or ideas. Think of several common words. What does each word stand for?
Another important way that humans communicate is with facial expressions. Look at the faces of the young children
in Figure 15.21 . Can you tell from their faces what the children are feeling?
Humans also use gestures to communicate. What are people communicating when they shrug their shoulders? When
they shake their head? These are just a few examples of the ways that humans share information without using words.
FIGURE 15.21
What does this girl&#8217 s face say
about how she is feeling
Social Behavior
Why is animal communication important? Without it, animals would not be able to live together in groups. Animals
that live in groups with other members of their species are called social animals. Social animals include many
species of insects, birds, and mammals. Specific examples of social animals are ants, bees, crows, wolves, and
humans. To live together with one another, these animals must be able to share information.
Highly Social Animals
Some species of animals are very social. In these species, members of the group depend completely on one another.
Different animals within the group have different jobs. Therefore, group members must work together for the good
of all. Most species of ants and bees are highly social animals.
Ants, like those in Figure 15.22 , live together in large groups called colonies. A colony may have millions of ants.
All of the ants in the colony work together as a single unit. Each ant has a specific job. Most of the ants are workers.
Their job is to build and repair the colony’s nest. Worker ants also leave the nest to find food for themselves and
other colony members. The workers care for the young as well. Other ants in the colony are soldiers. They defend
the colony against predators. Each colony also has a queen. Her only job is to lay eggs. She may lay millions of
eggs each month. A few ants in the colony are called drones. They are the only male ants in the colony. Their job is
to mate with the queen.
FIGURE 15.22
The ants in this picture belong to
the same colony. They have left the
colony&#8217 s nest to search for food.
Honeybees and bumblebees also live in colonies. A colony of honeybees is shown in Figure ?? don’t purge me.
Each bee in the colony has a particular job. Most of the bees are workers. Young worker bees clean the colony’s
hive and feed the young. Older worker bees build the waxy honeycomb or guard the hive. The oldest workers leave
the hive to find food. Each colony usually has one queen that lays eggs. The colony also has a small number of male
drones. They mate with the queen.
Ants, bees, and other social animals must cooperate. Cooperation means working together with others. Members
of the group may cooperate by sharing food. They may also cooperate by defending each other. Look at the ants in
Figure 15.24 . They show clearly why cooperation is important. A single ant would not be able to carry this large
insect back to the nest to feed the other ants. With cooperation, the job is easy.
Animals in many other species cooperate. For example, lions live in groups called prides. A lion pride is shown
in Figure 15.25 . All the lions in the pride cooperate. Male lions work together to defend the other lions in the
pride. Female lions work together to hunt. Then they share the meat with other pride members. Another example is
meerkats. Meerkats are small mammals that live in Africa. They also live in groups and cooperate with one another.
For example, young female meerkats act as babysitters. They take care of the baby meerkats while their parents are
away looking for food.
FIGURE 15.23
All the honeybees in this colony work
together. Each bee has a certain job
to perform. The bees are gathered together to fly to a new home. How do you
think they knew it was time to gather together
FIGURE 15.24
These ants are cooperating. By working together they are able to move this
much larger insect prey back to their
nest. At the nest they will share the insect with other ants that do not leave the
FIGURE 15.25
Members of this lion pride work together. Males cooperate by defending
the pride. Females cooperate by hunting and sharing the food.
Mating Behavior
Some of the most important animal behaviors involve mating. Mating is the pairing of an adult male and female
to produce young. Adults that are most successful at attracting a mate are most likely to have offspring. Traits that
help animals attract a mate and have offspring increase their fitness. As the genes that encode these traits are passed
to the next generation, the traits will become more common in the population.
Courtship Behaviors
In many species, females choose the male they will mate with. For their part, males try to be chosen as mates. They
show females that they would be a better mate than the other males. To be chosen as a mate, males may perform
courtship behaviors. These are special behaviors that help attract a mate. Male courtship behaviors get the attention
of females and show off a male’s traits.
Different species have different courtship behaviors. Remember the peacock raising his tail feathers in Figure 15.20
? This is an example of courtship behavior. The peacock is trying to impress females of his species with his beautiful
Another example of courtship behavior in birds is shown in Figure 15.26 . This bird is called a blue-footed booby.
He is doing a dance to attract a female for mating. During the dance, he spreads out his wings and stamps his feet on
the ground. You can watch a video of a blue-footed booby doing his courtship dance at:
Courtship behaviors occur in many other species. For example, males in some species of whales have special
mating songs to attract females as mates. Frogs croak for the same reason. Male deer clash antlers to court females. Male jumping spiders jump from side to side to attract mates. To see a video of a jumping spider courting a mate, go to:
Courtship behaviors are one type of display behavior. A display behavior is a fixed set of actions that carries a
specific message. Although many display behaviors are used to attract mates, some display behaviors have other
purposes. For example, display behaviors may be used to warn other animals to stay away, as you will read below.
FIGURE 15.26
This blue-footed booby is a species of
sea bird. The male pictured here is doing a courtship &#8220 dance.&#8221
He is trying to attract a female for mating.
Caring for the Young
In most species of birds and mammals, one or both parents care for their offspring. Caring for the young may include
making a nest or other shelter. It may also include feeding the young and protecting them from predators. Caring
for offspring increases their chances of surviving.
Birds called killdeers have an interesting way to protect their chicks. When a predator gets too close to her nest, a
mother killdeer pretends to have a broken wing. The mother walks away from the nest holding her wing as though
it is injured. This is what the killdeer in Figure 15.27 is doing. The predator thinks she is injured and will be easy
prey. The mother leads the predator away from the nest and then flies away.
FIGURE 15.27
This mother killdeer is pretending she
has a broken wing. She is trying to attract a predator&#8217 s attention in order to protect her chicks. This behavior
puts her at risk of harm. How can it increase her fitness
In most species of mammals, parents also teach their offspring important skills. For example, meerkat parents teach
their pups how to eat scorpions without being stung. A scorpion sting can be deadly, so this is a very important skill.
Teaching the young important skills makes it more likely that they will survive.
Defending Territory
Some species of animals are territorial. This means that they defend their area. The area they defend usually contains
their nest and enough food for themselves and their offspring. A species is more likely to be territorial if there is not
very much food in their area.
Animals generally do not defend their territory by fighting. Instead, they are more likely to use display behavior.
The behavior tells other animals to stay away. It gets the message across without the need for fighting. Display
behavior is generally safer and uses less energy than fighting.
Male gorillas use display behavior to defend their territory. They pound on their chests and thump the ground with
their hands to warn other male gorillas to keep away from their area. The robin in Figure 15.28 is also using display
behavior to defend his territory. He is displaying his red breast to warn other robins to stay away.
FIGURE 15.28
The red breast of this male robin is easy
to see. The robin displays his bright red
chest to defend his territory. It warns
other robins to keep out of his area.
Some animals deposit chemicals to mark the boundary of their territory. This is why dogs urinate on fire hydrants
and other objects. Cats may also mark their territory by depositing chemicals. They have scent glands in their face.
They deposit chemicals by rubbing their face against objects.
Cycles of Behavior
Many animal behaviors change in a regular way. They go through cycles. Some cycles of behavior repeat each year.
Other cycles of behavior repeat every day.
Yearly Cycles
An example of a behavior with a yearly cycle is hibernation. Hibernation is a state in which an animal’s body
processes are slower than usual and its body temperature falls. An animal uses less energy than usual during hibernation. This helps the animal survive during a time of year when food is scarce. Hibernation may last for weeks or
months. Animals that hibernate include species of bats, squirrels, and snakes.
Most people think that bears hibernate. In fact, bears do not go into true hibernation. In the winter, they go into a
deep sleep. However, their body processes do not slow down very much. Their body temperature also remains about
the same as usual. Bears can be awakened easily from their winter sleep.
Another example of a behavior with a yearly cycle is migration. Migration is the movement of animals from one
place to another. Migration is an innate behavior that is triggered by changes in the environment. For example,
animals may migrate when the days get shorter in the fall. Migration is most common in birds, fish, and insects.
In the Northern Hemisphere, many species of birds, including robins and geese, travel south for the winter. They
migrate to areas where it is warmer and where there is more food. They return north in the spring. A flock of
migrating geese is shown in Figure 15.29 .
FIGURE 15.29
These geese are flying south for the
winter. Flocks of geese migrate in Vshaped formations.
Some animals migrate very long distances. The map in Figure 15.30 shows the migration route of a species of hawk
called Swainson’s hawk. About how many miles do the hawks travel from start to finish? Are you surprised that
birds migrate that far? Some species of birds migrate even farther.
Birds and other migrating animals follow the same routes each year. How do they know where to go? It depends on
the species. Some animals follow landmarks, such as rivers or coastlines. Other animals are guided by the position
of the sun, the usual direction of the wind, or other clues in the environment.
FIGURE 15.30
son&#8217 s hawk starts in North
America and ends in South America.
Scientists learned their migration route
by attaching tiny tracking devices to the
birds. The birds were then tracked by
satellite. On the migration south the
hawks travel almost 5 000 miles from
start to finish.
Daily Cycles
Many animal behaviors change at certain times of day, day after day. For example, most animals go to sleep when
the sun sets and wake up when the sun rises. Animals that are active during the daytime are called diurnal. Some
animals do the opposite. They sleep all day and are active during the night. These animals are called nocturnal.
Animals may eat and drink at certain times of day, as well. Humans have daily cycles of behavior, too. Most people
start to get sleepy after dark and have a hard time sleeping when it is light outside. Daily cycles of behavior are
called circadian rhythms.
In many species, including humans, circadian rhythms are controlled by a tiny structure called the biological clock.
This structure is located in a gland at the base of the brain. The biological clock sends signals to the body. The
signals cause regular changes in behavior and body processes. The amount of light entering the eyes controls the
biological clock. That’s why the clock causes changes that repeat every 24 hours.
Lesson Summary
Communication is any way that animals share information.
Social animals live together in groups. These animals need to cooperate with one another.
Some of the most important animal behaviors involve attracting mates. Others involve caring for offspring.
Some animals defend the area where they live from other animals.
Many animal behaviors occur in cycles that repeat daily or yearly.
Review Questions
1. List two ways that animals communicate.
2. Describe how ants in a colony cooperate.
3. What is courtship behavior?
Apply Concepts
4. Why do male dogs urinate on fire hydrants and other objects?
5. Give an example of a circadian rhythm.
6. How do ants use chemicals to communicate?
7. What is the advantage of animals using display behavior instead of fighting to defend their territory?
8. What is migration, and why do animals migrate?
Critical Thinking
9. Explain how courtship behaviors could evolve.
10. How do adult animals increase their own fitness by teaching skills to their young?
Further Reading / Supplemental Links
Bernard Stonehouse and Esther Bertram. The Truth about Animal Communications. Tangerine Press, 2003.
Betty Tatham. How Animals Communicate. Franklin Watts. 2004.
Etta Kaner. Animal Groups (Animal Behavior). Tandem Library, 2004.
Pamela Hickman. Animals and Their Mates: How Animals Attract, Fight for, and Protect Each Other. Kids
Can Press, Ltd., 2004.
• Susan Glass. Staying Alive: Regulation and Behavior. Perfection Learning, 2005.
• mnkylab/media/vervetcalls.html
Points to Consider
Next we start discussing the human body.
• The biological clock located just below the human brain controls behaviors such as the sleep-wake cycle. The
brain is part of the nervous system. Can you name some other body systems found in humans?
• Which body system includes the bones? Which system includes the muscles? What do bones and muscles
16MS Skin, Bones, and Muscles
What are human skeletons made out of? Bone. What is connected to the bone? Muscle. What layer rests on top of
muscle to protect your body? Skin. Bones, muscle, and skin form the foundation of your body.
The person in the above figure is moving. How do his bones help him to move? How are his muscles working
with his bones? What would happen if the he did not have skin? What would happen if his bones were removed?
Human bodies cannot work without a collection of different tissues and organs. If you remove one, the others will
not operate in the same way.
While reading this chapter, think about the individual functions of bones, muscles, and skin. But also remember to
consider how they work together to form a whole human.
Organization of Your Body
Lesson Objectives
List the levels of organization in the human body.
Identify the four types of tissues that make up the body.
Identify 12 organ systems.
Describe how organs and organ systems work together to maintain homeostasis.
Check Your Understanding
• What is a cell?
• What are some of the differences between a prokaryotic cell and an eukaryotic cell?
• What are some of the basic functions of animal cells?
cardiovascular system The organ system that is made up of the heart, the blood vessels, and the blood.
connective tissue Tissue that is made up of different types of cells that are involved in structure and support of the
body; includes blood, bone, tendons, ligaments, and cartilage.
epithelial tissue A tissue that is composed of layers of tightly packed cells that line the surfaces of the body;
examples of epithelial tissue include the skin, the lining of the mouth and nose, and the lining of the digestive
feedback regulation Control of a biological process based on the effect of a stimulus.
muscular tissue Tissue that is composed of cells that have filaments that move past each other and change the size
of the cell. There are three types of muscle tissue: smooth muscle, skeletal muscle, and cardiac muscle.
negative feedback loop When the response to a stimulus decreases the effect of the original stimulus.
nervous tissue Tissue composed of nerve cells (neurons) and related cells.
positive feedback loop When the response to a stimulus increases the original stimulus.
An introduction to the human body can be viewed at (1:45).
Click image to the left for more content.
Cells, Tissues, Organs
The men in Figure 16.1 just jumped into freezing icy water. They are having fun, but imagine how cold they must
feel! What happens to their bodies when one moment they are warm and the next they are freezing? If their bodies
are working right, they will begin to shiver. Shivering helps the body return to a stable temperature.
The ability of the body to maintain a stable internal environment in response to change is called homeostasis. Homeostasis allows your body to adapt to change. Change might be from jumping into cold water or running in hot
weather. Or it might be from not getting enough food when you are hungry. Homeostasis is a very important
characteristic of living things.
The bodies of these swimmers are
working hard to maintain homeostasis
while they are in the icy pool water. Otherwise their life processes would stop
working as soon as they got into the water.
Homeostasis and Cells
Cells are the most basic units of life in your body. They must do many jobs to maintain homeostasis, but each cell
does not have to do every job. Cells have specific jobs to maintain homeostasis. For example, nerve cells move
electrical messages around the body, and white blood cells patrol the body and attack invading bacteria.
There are many additional different types of cells. Other cells include red blood cells, skin cells, cells that line the
inside of your stomach, and muscle cells.
Groups of Cells Form Tissues
Cells are grouped together to carry out specific functions. A group of cells that work together is called a tissue. Your
body has four main types of tissues, as do the bodies of other animals. These tissues make up all structures and
contents of your body. An example of each tissue type is shown in Figure 16.2 .
Your body has four main types of tissue nervous tissue epithelial tissue connective tissue and muscle tissue.
They are found throughout your body.
a. Epithelial tissue is made up of layers of tightly packed cells that line the surfaces of the body. Examples of
epithelial tissue include the skin, the lining of the mouth and nose, and the lining of the digestive system.
b. Connective tissue is made up of many different types of cells that are all involved in structure and support of
the body. Examples include tendon, cartilage, and bone. Blood is also classified as a specialized connective
c. Muscle tissue is made up of bands of cells that contract and allow bodies to move. There are three types of
muscle tissue: smooth muscle, skeletal muscle, and cardiac muscle.
d. Nervous tissue is made up of the nerve cells that together form the nervous system. Nervous tissue is found
in nerves, the spinal cord, and the brain.
Groups of Tissues Form Organs
A single tissue alone cannot do all the jobs that are needed to keep you alive and healthy. Two or more tissues
working together can do a lot more. An organ is a structure made of two or more tissues that work together. The
heart, shown in Figure 16.3 , is made up of the four types of tissues.
The four different tissue types work together in the heart as they do in the
other organs.
Groups of Organs Form Organ Systems
Your heart pumps blood around your body. But how does your heart get blood to and from every cell in your body?
Your heart is connected to blood vessels such as veins and arteries. Organs that work together form an organ system.
Together, your heart, blood, and blood vessels form your cardiovascular system.
What other organ systems can you think of?
Organ Systems Work Together
Your body’s 12 organ systems are shown in Table 16.2 . Your organ systems do not work alone in your body. They
must all be able to work together to maintain homeostasis.
For example, when the men in Figure 16.1 jumped into the cold water, their integumentary system (skin), cardiovascular system, muscular system, and nervous system worked quickly together to ensure the icy water did not cause
harm to their bodies.
For example, the nervous system sent nerve messages from the skin to tell the cardiovascular system to reduce the
blood flow to the skin. Blood flow is then increased to the internal organs and large muscles to help keep them warm
and supply them with oxygen. The nervous system also sent messages to the respiratory system to breathe faster.
This allows for more oxygen to be delivered by the blood to the muscular system.
One of the most important functions of organ systems is to provide cells with oxygen and nutrients and to remove
toxic waste products such as carbon dioxide. A number of organ systems, including the cardiovascular and respiratory systems, all work together to do this.
TABLE 16.1: Major Organ Systems of the Human Body
Organ System
Major Tissues and Organs
Heart; blood vessels; blood
Lymph nodes; lymph vessels
Esophagus; stomach; small intestine; large intestine
Pituitary gland, hypothalamus;
adrenal glands; Islets of Langerhans; ovaries; testes
Skin, hair, nails
Cardiac (heart) muscle; skeletal
muscle; smooth muscle; tendons
Brain, spinal cord; nerves
Female: uterus; vagina; fallopian
tubes; ovaries Male: penis; testes;
seminal vesicles
Trachea, larynx, pharynx, lungs
Bones, cartilage; ligaments
Kidneys; urinary bladder
Skin; bone marrow; spleen; white
blood cells
Transports oxygen, hormones and
nutrients to the body cells. Moves
wastes and carbon dioxide away
from cells
Defense against infection and disease, moves lymph between tissues
and the blood stream
Digests foods and absorbs nutrients,
minerals, vitamins, and water
Hormones communicate between
cells to maintain homeostasis
Protection from injury and water
loss; physical defense against infection by microorganisms; temperature control
Movement, support, heat production
Collects, transfers and processes information
Production of gametes (sex cells)
and sex hormones; production of
Brings air to sites where gas exchange can occur between the blood
and cells (around body) or blood
and air (lungs)
Supports and protects soft tissues of
body; movement at joints; produces
blood cells; stores minerals
Removes extra water, salts, and
waste products from blood and
body; control of pH; controls water
and salt balance
Defense against diseases
TABLE 16.2: Major Organ Systems of the Human Body
Organ System
TABLE 16.2: (continued)
Organ System
You can watch overviews of the human organ systems at the link below.
Click image to the left for more content.
Homeostasis and Feedback Regulation
As described above, homeostasis is an organism’s ability to keep a constant internal environment. Keeping a stable
internal environment requires constant adjustments as conditions change inside and outside of cells.
The endocrine system plays an important role in homeostasis because hormones, which are the messengers of the
endocrine system, regulate the activity of body cells. The release of hormones into the blood is controlled by a
stimulus, or signal. For example, the stimulus either causes an increase or a decrease in the amount of hormone
released. Then, the response to the signal changes the internal conditions and may itself become a new stimulus.
This kind of control is called feedback regulation or a feedback loop.
Feedback regulation occurs when the response to a stimulus has an effect of some kind on the original stimulus.
The type of response determines what the feedback is called. Take the men jumping in the water as an example.
When the nervous system reduced blood flow to the skin, this was caused by a feedback loop sensing the change in
environment. Feedback loops return body systems back to "normal."
A negative feedback loop is one in which the response to a stimulus decreases the effect of the original stimulus.
A positive feedback loop is one in which the response to a stimulus increases the original stimulus. These are
explained in more detail below.
Thermoregulation: A Negative Feedback Loop
Negative feedback is the most common feedback loop in the body. Negative feedback decreases the effect of a
stimulus on the body (Figure 16.4 ). For instance, if you get stuck in a smoky environment during a fire, the amount
of carbon dioxide in your body will increase. In the negative feedback loop, your lungs will be signaled to increase
your breathing rate and exhale more carbon dioxide. The effect is to reduce the amount of carbon dioxide in your
body. You can remember that this is a negative feedback loop because you are decreasing the effect of the stimulus.
Thermoregulation is another example of negative feedback. When body temperature rises, receptors in the skin and
the brain sense the temperature change. The temperature change triggers a command from the brain. This command
causes a response — the skin makes sweat and blood vessels near the skin surface dilate. This response helps
decrease body temperature.
Positive feedback is less common than negative feedback. Positive feedback acts to increase the effect of a stimulus.
An example of positive feedback is milk production. As the baby drinks its mother’s milk, nerve messages from
the mammary glands cause a hormone, prolactin, to be released. The more the baby suckles, the more prolactin is
released, which causes more milk to be produced.
Feedback Regulation.
Lesson Summary
The levels of organization in the human body include: cells, tissues, organs, and organ systems.
A tissue is a group of cells that work together.
An organ is made of two or more tissues that work together.
Organs that work together make up organ systems.
There are four tissue types in the body: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.
There are 12 major organ systems in the body.
Organs and organ systems work together to maintain homeostasis.
Review Questions
1. What is homeostasis?
2. What are the four levels of organization in an organism?
3. List the four types of tissues that make up the human body.
Apply Concepts
4. What is the difference between a tissue and an organ?
5. Identify the organ system to which the following organs belong: skin, stomach, brain, lungs, and heart.
6. Give an example of how two organ systems work together to maintain homeostasis.
Critical Thinking
7. A classmate says that all four tissue types are never found together in an organ. Do you agree with your classmate?
Explain your answer.
8. Why do you think an organ is able to do many more jobs than a single tissue?
Further Reading / Supplemental Links
Points to Consider
The first system we will discuss is the integumentary system.
• What organs do you think makes up the integumentary system?
• What other body systems might the integumentary system work with to maintain homeostasis?
The Integumentary System
Lesson Objectives
List the functions of skin.
Describe the structure of skin.
Describe the structure of hair and nails.
Identify two types of skin problems.
Describe two ways to take care of your skin.
Check Your Understanding
• What is homeostasis?
• What is epithelial tissue?
dermis The layer of skin directly under the epidermis; made of a tough connective tissue that contains the protein
epidermis The outermost layer of the skin; forms the waterproof, protective wrap over the body’s surface; made
up of many layers of epithelial cells.
integumentary system The outer covering of the body; made up of the skin, hair, and nails.
keratin Tough, waterproof protein that is found in epidermal skin cells, nail, and hair.
melanin The brownish pigment that gives skin and hair their color.
oil gland Skin organ that secretes an oily substance, called sebum, into the hair follicle.
sunburn A burn to the skin that is caused by overexposure to UV radiation from the sun’s rays or tanning beds.
sweat gland Gland that opens to the skin surface through skin pores; found all over the body; secretes sweat.
Your Skin and Homeostasis
Did you know that you see the largest organ in your body every day? You wash it, dry it, cover it up to stay warm
or uncover it to cool off. In fact, you see it so often it is easy to forget the important role your skin plays in keeping
you healthy.
Your skin is part of your integumentary system (Figure 16.5 ), which is the outer covering of your body. The
integumentary system is made up of your skin, hair, and nails. Your integumentary system has many roles in
homeostasis, including protection, the sense of touch, and controlling body temperature.
Skin acts as a barrier that stops water
and other things like soap and dirt from
getting into your body.
Functions of Skin
Your skin covers the entire outside of your body. Your skin is your body’s largest organ, yet it is only about 2
millimeters thick. It has many important functions. The skin:
• Provides a barrier. It keeps organisms that could harm the body out. It stops water from leaving the body, and
stops water from getting into the body.
• Controls body temperature. It does this by making sweat, a watery substance that cools the body when it
• Gathers information about your environment. Special nerve endings in your skin sense heat, pressure, cold
and pain.
• Helps the body get rid of some types of waste, which are removed in sweat.
• Acts as a sun block. A chemical called melanin is made by certain skin cells when they are exposed to sunlight.
Melanin blocks sun light from getting to deeper layers of skin cells, which are easily damaged by sunlight.
Structure of Skin
Your skin is always exposed to your external environment, so it gets cut, scratched, and worn down. You also
naturally shed many skin cells every day. Your body replaces damaged or missing skin cells by growing more of
them. Did you know that the layer of skin you can see is actually dead? The dead cells are filled with a tough,
waterproof protein called keratin. As the dead cells are shed or removed from the upper layer, they are replaced by
the skin cells below them.
As you can see in Figure 16.6 , two different layers make up the skin — the epidermis and the dermis. A fatty layer,
called subcutaneous tissue, lies under the dermis, but it is not part of your skin.
Skin is made up of two layers the epidermis on top and the dermis below.
The tissue below the dermis is called
the hypodermis but it is not part of the
The color, thickness and texture of skin vary over the body. There are two general types of skin:
a. Thin and hairy, which is the most common type on the body.
b. Thick and hairless, which is found on parts of the body that experience a lot of contact with the environment,
such as the palms of the hands or the soles of the feet.
The Epidermis
The epidermis is the outermost layer of the skin. It forms the waterproof, protective wrap over the body’s surface.
The epidermis is divided into several layers of epithelial cells. The epithelial cells are formed by mitosis in the
lowest layer. These cells move up through the layers of the epidermis to the top. Although the top layer of epidermis
is only about as thick as a sheet of paper, it is made up of 25 to 30 layers of cells.
The epidermis also contains cells that produce melanin. Melanin is the brownish pigment that gives skin and hair
their color. Melanin-producing cells are found in the bottom layer of the epidermis.
The epidermis does not have any blood vessels. The lower part of the epidermis receives blood by diffusion from
blood vessels of the dermis.
The Dermis
The dermis is the layer of skin directly under the epidermis. It is made of a tough connective tissue that contains
the protein collagen. Collagen is a long, fiber-like protein that is very strong. The dermis is tightly connected to the
epidermis by a thin wall of collagen fibers.
As you can see in Figure 16.6 , the dermis contains hair follicles, sweat glands, oil glands, and blood vessels. It also
holds many nerve endings that give you your sense of touch, pressure, heat, and pain.
Do you ever notice how your hair stands up when you are cold or afraid? Tiny muscles in the dermis pull on hair
follicles which cause hair to stand up. The resulting little bumps in the skin are commonly called "goosebumps,"
shown in Figure 16.7 .
Goosebumps are caused by tiny muscles in the dermis that pull on hair follicles which causes the hairs to stand up
Oil Glands and Sweat Glands
Glands and follicles open out into the epidermis, but they start in the dermis. Oil glands release, or secrete, an oily
substance, called sebum, into the hair follicle. An oil gland is shown in Figure 16.6 . Sebum “waterproofs” hair and
the skin surface to prevent them from drying out. It can also stop the growth of bacteria on the skin. It is odorless,
but the breakdown of sebum by bacteria can cause odors. If an oil gland becomes plugged and infected, it develops
into a pimple. Up to 85% of teenagers get pimples, which usually go away by adulthood. Frequent washing can help
decrease the amount of sebum on the skin.
Sweat glands open to the skin surface through skin pores. They are found all over the body. Evaporation of sweat
from the skin surface helps to lower skin temperature. This is why sweat can help maintain homeostasis. The skin
also releases excess water, salts, and other wastes in sweat. A sweat gland is shown in Figure 16.6 .
Nails and Hair
Nails and hair are made of the same types of cells that make up skin. Hair and nails contain the tough protein keratin.
Fingernails and toenails both grow from nail beds. A nailbed is thickened to form a lunula, or little moon, which
you can see in Figure 16.8 . Cells forming the nail bed are linked together to form the nail. As the nail grows, more
cells are added at the nail bed. Older cells get pushed away from the nail bed and the nail grows longer. There are
no nerve endings in the nail, which is a good thing, otherwise cutting your nails would hurt a lot!
Nails act as protective plates over the fingertips and toes. Fingernails also help in sensing the environment. The
area under your nail has many nerve endings, which allow you to receive more information about objects you touch.
Nails are made up of many different parts, as shown in Figure 16.8 .
The structure of fingernails is similar
to toenails. The free edge is the part
of the nail that extends past the finger beyond the nail plate. The nail
plate is what we think of when we say
&#8220 nail &#8221 the hard portion
made of the tough protein keratin. The
lunula is the crescent shaped whitish
area of the nail bed. The cuticle is the
fold of skin at the end of the nail.
Hair sticks out from the epidermis, but it grows from the dermis, as shown in Figure 16.9 . Hair is also made of
keratin, the same protein that makes up skin and nails. Hair grows from inside the hair follicle. New cells grow in
the bottom part of the hair, called the bulb. Older cells get pushed up, and the hair grows longer. Similar to nails and
skin, the cells that make up the hair strand are dead and filled with keratin.
Hair color is caused by different types of melanin in the hair cells. In general, the more melanin in the cells, the
darker the hair color; the less melanin, the lighter the hair color.
Hair hair follicle and oil glands. The
oil called sebum helps to prevent water
loss from the skin.
Hair helps to keep the body warm. When you feel cold, your skin gets a little bumpy. These bumps are caused by
tiny muscles that pull on the hair, causing the hair to stick out. The erect hairs help to trap a thin layer of air that is
warmed by body heat. In mammals that have much more hair than humans, the hair traps a layer of warm air near
the skin and acts like warm blanket. Hair also protects the skin from ultraviolet (UV) radiation from the sun.
Hair also acts as a filter. Nose hair helps to trap particles in the air that may otherwise travel to the lungs. Eyelashes
shield eyes from dust and sunlight. Eyebrows stop salty sweat and rain from flowing into the eye.
Keeping Skin Healthy
Some sunlight is good for health. Vitamin D is made in the skin when it is exposed to sunlight. But getting too much
sun can be unhealthy. A sunburn is a burn to the skin that is caused by overexposure to UV radiation from the sun’s
rays or tanning beds.
Light-skinned people, like the girl in Figure 16.10 , get sunburned more quickly than people with darker skin. This
is because melanin in the skin acts as a natural sunblock that helps to protect the body from UV radiation. When
exposed to UV radiation, certain skin cells make melanin, which causes skin to tan. Children and teens who have
gotten sunburned are at a greater risk of developing skin cancer later in life.
Long-term exposure to UV radiation is the leading cause of skin cancer. About 90 percent of skin cancers are linked
to sun exposure. UV radiation damages the genetic material of skin cells. This damage can cause the skin cells to
grow out of control and form a tumor. Some of these tumors are very difficult to cure. For this reason you should
always wear sunscreen with a high sun protection factor (SPF), a hat, and clothing when out in the sun. As people
age, their skin gets wrinkled. Wrinkles are caused mainly by UV radiation and by the loosening of the connective
tissue in the dermis due to age.
FIGURE 16.10
Sunburn is caused by overexposure to
UV rays. Getting sunburned as a child or
a teen especially sunburn that causes
blistering increases the risk of developing skin cancer later in life.
Bathing and Skin Hygiene
During the day, your skin can collect many different things. Sweat, oil, dirt, dust, and dead skin cells can build up
on the skin surface. If not washed away, the mix of sweat, oil, dirt, and dead skin cells can encourage the excess
growth of bacteria. These bacteria feed on these substances and cause a smell that is commonly called body odor.
Dirty skin is also more prone to infection. Bathing every day helps to remove dirt, sweat and extra skin cells, and
helps to keep your skin clean and healthy.
Your skin can heal itself even after a large cut. Cells that are damaged or cut away are replaced by cells that grow
in the bottom layer of the epidermis and the dermis. When an injury cuts through the epidermis into the dermis,
bleeding occurs. A blood clot and scab soon forms. After the scab is formed, cells at the bottom of the epidermis
begin to divide by mitosis and move to the edges of the scab. A few days after the injury, the edges of the wound are
pulled together.
If the cut is large enough, the production of new skin cells will not be able to heal the wound. Stitching the edges of
the injured skin together can help the skin to repair itself. The person in Figure 16.11 had a large cut that needed to
be stitched together. When the damaged cells and tissues have been replaced, the stitches can be removed.
Lesson Summary
Skin acts as a barrier that keeps particles and water out of the body.
The skin helps to cool the body in hot temperatures, and keep the body warm in cool temperatures.
Skin is made up of two layers, the epidermis and the dermis.
Pimples occur when the skin produces too much sebum.
Hair and nails are made of keratin, the same protein as skin.
FIGURE 16.11
Sewing the edges of a large cut together
allows the body to repair the damaged
cells and tissues and heal the tear in the
Nails grow from nail beds and hairs grow from hair follicles in the skin.
Skin cancer can be caused by excess exposure to ultraviolet light from the sun or tanning beds.
Frequent bathing helps keep the skin clean and healthy.
Wearing sun block and a hat when outdoors can help prevent skin cancer.
Review Questions
1. Identify two functions of skin.
2. How does the integumentary system help maintain homeostasis?
3. What are the two layers of the skin?
4. Identify the layer of skin from which hair grows.
5. Name two functions of nails.
6. Name two functions of hair.
Apply Concepts
7. In what way are hair and nails similar to skin?
8. The skin makes too much sebum, what type of skin problem might this cause?
9. How does washing your skin help to keep you healthy?
10. Why are stitches sometimes needed if a person gets a deep or long cut in their skin?
Critical Thinking
11. The World Health Organization recommends that no person younger than 18 years old use a tanning bed. Why
do you think using a tanning bed is not recommended?
Further Reading / Supplemental Links
Points to Consider
Next we turn to the skeletal system.
• How might what you eat affect your bones?
• What do you think is the most important function of your skeletal system?
The Skeletal System
Lesson Objectives
Identify the main tissues and organs of the skeletal system.
List four functions of the skeletal system.
Describe three movable joints.
Identify two nutrients that are important for a healthy skeletal system.
Describe two skeletal system injuries.
Check Your Understanding
• What is an organ system?
• What is connective tissue?
ball and socket joint Joint structure in which the ball-shaped surface of one bone fits into the cuplike depression
in another bone; examples include the shoulder and hip joints.
bone marrow Soft connective tissue found inside many bones; site of blood cell formation.
cartilage Smooth covering found at the end of bones; made of tough collagen protein fibers; creates smooth
surfaces for the easy movement of bones against each other.
fracture Bone injury, often called a break; usually caused by excess bending stress on bone.
gliding joint Joint structure that allows one bone to slide over the other; examples includes the joints in the wrists
and ankles.
hinge joint Joint structure in which the ends of bones are shaped in a way that allows motion in two directions
only (forward and backward); examples include the knees and elbows.
joint Point at which two or more bones meet.
ligament Fibrous tissue that connects bones to other bones; made of tough collagen fibers.
movable joint Most mobile type of joint; the most common type of joint in the body.
pivot joint Joint structure in which the end on one bone rotates within a ring-type structure which can be made
partly of bone and partly of ligament.
skeletal system Body system that is made up of bones, cartilage, and ligaments.
skeleton Sturdy scaffolding of bones and cartilage that is found inside vertebrates.
sprain A ligament injury; usually caused by the sudden overstretching of a joint which causes tearing.
Your Skeleton
How important is your skeleton? Can you imagine your body without it? You would be a wobbly pile of muscle and
internal organs, and you would not be able to move.
Your skeleton is important for many different things. Bones are the main organs of the skeletal system. They are
made up of living tissue. Humans are vertebrates, which are animals that have a backbone. The sturdy set of bones
and cartilage that is found inside vertebrates is called a skeleton.
The adult human skeleton has 206 bones, some of which are named in Figure 16.12 . Strangely, even though they
are smaller, the skeletons of babies and children have many more bones and more cartilage than adults have. As a
child grows, these “extra” bones grow into each other, and cartilage slowly hardens to become bone tissue.
Living bones are full of life. They contain many different types of tissues. Cartilage is found at the end of bones
and is made of tough protein fibers called collagen. Cartilage creates smooth surfaces for the movement of bones
that are next to each other, like the bones of the knee.
Ligaments are made of tough protein fibers and connect bones to each other. Your bones, cartilage, and ligaments
make up your skeletal system.
Functions of Bones
Your skeletal system gives shape and form to your body, but it is also important in maintaining homeostasis. The
main functions of the skeletal system include:
• Support. The skeleton supports the body against the pull of gravity, meaning you don’t fall over when you
stand up. The large bones of the lower limbs support the rest of the body when standing.
• Protection. The skeleton supports and protects the soft organs of the body. For example, the skull surrounds
the brain to protect it from injury. The bones of the rib cage help protect the heart and lungs.
• Movement. Bones work together with muscles to move the body.
• Making blood cells. Blood cells are mostly made inside certain types of bones.
• Storage. Bones store calcium. They contain more calcium than any other organ. Calcium is released by the
bones when blood levels of calcium drop too low. The mineral phosphorus is also stored in bones.
Structure of Bones
Bones are organs. Recall that organs are made up of two or more types of tissues. Bones come in many different
shapes and sizes, but they are all made of the same materials.
FIGURE 16.12
The skeletal system is made up of
bones cartilage and ligaments. The
skeletal system has many important
functions in your body.
The two main types of bone tissue are compact bone and spongy bone.
• Compact bone makes up the dense outer layer of bones.
• Spongy bone is found at the center of the bone, and is lighter and less dense than compact bone.
Bones look tough, shiny, and white because they are covered by a layer called the periosteum. Many bones also
contain a soft connective tissue called bone marrow. There are two types of bone marrow - red marrow and yellow
• Red marrow makes red blood cells, platelets, and most of the white blood cells for the body (discussed in the
Diseases and the Body’s Defenses chapter).
• Yellow marrow makes white blood cells.
The bones of newborn babies contain only red marrow. As children get older, some of their red marrow is replaced
by yellow marrow. In adults, red marrow is found mostly in the bones of the skull, the ribs, and pelvic bones.
Bones come in four main shapes. They can be long, short, flat, or irregular. Identifying a bone as long, short, flat, or
irregular is based on the shape of the bone, not the size of the bone. For example, both small and large bones can be
classified as long bones. The small bones in your fingers and the largest bone in your body, the femur, are all long
bones. The structure of a long bone is shown in Figure 16.13 .
FIGURE 16.13
Bones are made up of different types of
Bone Growth
Your skeleton begins growing very early in development. After only eight weeks of growth from a fertilized egg,
your skeleton has been formed by cartilage and other connective tissues.
At this point your skeleton is very flexible. After a few more weeks of growth, the cells that form hard bone begin
growing in the cartilage, and your skeleton begins to harden. Not all of the cartilage, however, is replaced by bone.
Cartilage remains in many places in your body, including your joints, your rib cage, your ears, and the tip of your
A baby is born with zones of cartilage in its bones that allow growth of the bones. These areas, called growth plate,
allow the bones to grow longer as the child grows. By the time the child reaches an age of about 18 to 25 years, all
of the cartilage in the growth plate has been replaced by bone. This stops the bone from growing any longer.
Even though bones stop growing in length in early adulthood, they can continue to increase in thickness throughout
life. This thickening occurs in response to strain from increased muscle activity and from weight-lifting exercises.
Joints and How They Move
A joint is a point at which two or more bones meet. There are three types of joints in the body:
a. Fixed joints do not allow any bone movement. Many of the joints in your skull are fixed (Figure 16.14 ).
b. Partly movable joints allow only a little movement. Your backbone has partly movable joints between the
vertebrae (Figure 16.15 ).
c. Movable joints allow movement.
Joints are a type of lever, which is a rigid object that is used to increase the amount of force put onto another object.
Can openers and scissors are examples of levers. Joints reduce the amount of energy that is spent moving the body
around. Just imagine how difficult it would be to walk about if you did not have knees!
FIGURE 16.14
The skull has fixed joints. Fixed joints do
not allow any movement of the bones
which protects the brain from injury.
Movable Joints
Movable joints are the most mobile joints of all. They are also the most common type of joint in your body. Your
fingers, toes, hips, elbows, and knees all provide examples movable joints. The surfaces of bones at movable joints
are covered with a smooth layer of cartilage. The space between the bones in a movable joint is filled with a liquid
FIGURE 16.15
The joints between your vertebrae are
partially movable.
called synovial fluid. Synovial fluid is a thick, stringy fluid that looks like egg white. The fluid gives the bone a
smooth cushion when they move at the joint. Four types of movable joints are shown below.
1. In a ball and socket joint, the ball-shaped surface of one bone fits into the cup-like shape of another. Examples
of a ball and socket joint include the hip, shown in Figure 16.16 , and the shoulder.
FIGURE 16.16
joint is a ball and socket joint. The
&#8220 ball&#8221 end of one bone
fits into the &#8220 socket&#8221 of
another bone. These joints can move in
many different directions.
2. In a hinge joint, the ends of the bones are shaped in a way that allows motion only in two directions, forward and
backward. Examples of hinge joints are the knees and elbows. A knee joint is shown in Figure 16.17 .
3. The pivot joint is formed by a process that rotates within a ring, the ring being formed partly of bone and partly
of ligament. An example of a pivot joint is the joint between the radius and ulna that allows you to turn the palm of
your hand up and down. A pivot joint is shown in Figure 16.18 .
FIGURE 16.17
Hinge Joint. The knee joint is a hinge
joint. Like a door hinge a hinge joint allows backward and forward movement.
4. A gliding joint is a joint which allows only gliding movement. The gliding joint allows one bone to slide over the
other. The gliding joint in your wrist allows you to flex your wrist. It also allows you to make very small side-to-side
motions. There are also gliding joints in your ankles.
FIGURE 16.18
Pivot Joint. The joint at which the radius
and ulna meet is a pivot joint. Movement
at this joint allows you to flip your palm
over without moving your elbow joint.
Keeping Bones and Joints Healthy
Your body depends on you to take care of it, just like you may take care of a plant or a dog. You can help keep your
bones and skeletal system healthy by eating well and getting enough exercise. Weight-bearing exercises help keep
bones strong. Weight-bearing exercises work against gravity. Such activities include basketball, tennis, gymnastics,
karate, running, and walking. When the body is exercised regularly by performing weight-bearing activity, bones
respond by adding more bone cells to increase their bone density.
Eating Well
Did you know that what you eat as a teenager can affect how healthy your skeletal system will be in 30, 40, and even
50 years? Calcium and vitamin D are two of the most important nutrients for a healthy skeletal system. Your bones
need calcium to grow properly. If you do not get enough calcium in your diet as a teenager, your bones may become
weak and break easily later in life.
Osteoporosis is a disease in which bones become lighter and more porous than they should be. Light and porous
bones are more likely to break, which can cause pain and prevent a person from walking. Two of the easiest ways
to prevent osteoporosis are eating a healthy diet that has the right amount of calcium and vitamin D, and to do some
sort of weight-bearing exercise every day. Foods that are a good source of calcium include milk, yogurt, and cheese.
Non-dairy sources of calcium include Chinese cabbage, kale, and broccoli. Many fruit juices, fruit drinks, tofu,
and cereals have calcium added to them. It is recommended that teenagers get 1300 mg of calcium every day. For
example, one cup of milk provides about 300 mg of calcium, or about 30% of the daily requirement. Other sources
of calcium are shown in Figure 16.19 .
FIGURE 16.19
There are many different sources of calcium. Getting enough calcium in your
daily diet is important for good bone
health. How many ounces of cheddar cheese would provide your recommended daily intake of calcium
Your skin makes vitamin D when exposed to sunlight. The pigment melanin in the skin acts like a filter that can
prevent the skin from making vitamin D. As a result, people with darker skin need more time in the sun than people
with lighter skin to make the same amount of vitamin D.
Fish is naturally rich in vitamin D. Vitamin D is added to other foods, including milk, soy milk, and breakfast cereals.
Teenagers are recommended to get 5 micrograms (200 IU) of vitamin D every day. A 3 12 -ounce portion of cooked
salmon provides 360 IU of vitamin D.
Bone Fractures
Even though they are very strong, bones can fracture, or break. Fractures can happen at different places on a bone.
They are usually caused by excess bending stress on the bone. Bending stress is what causes a pencil to break if you
bend it too far.
Soon after a fracture, the body begins to repair the break. The area becomes swollen and sore. Within a few days
bone cells travel to the break site and begin to rebuild the bone. It takes about two to three months before compact
and spongy bone form at the break site. Sometimes the body needs extra help in repairing a broken bone. In such a
case a surgeon will piece a broken bone together with metal pins. Moving the broken pieces together will help keep
the bone from moving, and give the body a chance to repair the break. A broken ulna has been repaired with pins in
Figure 16.20 .
FIGURE 16.20
The upper part of the ulna just above
the elbow joint is broken as you can see
in the X-ray at left. The x-ray at right was
taken after a surgeon inserted a system of pins and wires across the fracture to bring the two pieces of the ulna
into close proximity.
Cartilage Injuries
Osteoarthritis occurs when the cartilage at the ends of the bones breaks down. The break down of the cartilage leads
to pain and stiffness in the joint. Decreased movement of the joint because of the pain may lead to weakening of the
muscles that normally move the joint, and the ligaments surrounding the joint may become looser. Osteoarthritis is
the most common form of arthritis. It has many causes, including aging, sport injuries, fractures, and obesity.
Ligament Injuries
Recall that a ligament is a short band of tough connective tissue that connects bones together to form a joint. Ligaments can get injured when a joint gets twisted or bends too far. The protein fibers that make up a ligament can get
strained or torn, causing swelling and pain. Injuries to ligaments are called sprains. Ankle sprains are a common
type of sprain. A sprain of the anterior cruciform ligament (ACL), a small ligament in the knee, is a common injury
among athletes. Ligament injuries can take a long time to heal. Treatment of the injury includes rest and special
exercises that are developed by a physical therapist.
Preventing Injuries
Preventing injuries to your bones and ligaments is easier and much less painful than treating an injury. Wearing the
correct safety equipment when performing activities that require such equipment can help prevent many common
injuries. For example, wearing a bicycle helmet can help prevent a skull injury if you fall. Warming up and cooling
down properly can help prevent ligament and muscle injuries. Stretching before and after activity also helps prevent
injuries. Stretching can improve your posture, and helps prevent some aches and pains associated with tight muscles.
Lesson Summary
Bones, cartilage, and ligaments make up the skeletal system.
The skeleton supports the body against the pull of gravity.
The skeleton provides a framework that supports and protects the soft organs of the body.
Bones work together with muscles to move the body.
Blood cells are mostly made inside the bone marrow.
There are three types of joints in the body: fixed, partly movable, and movable.
Calcium and vitamin D are two of the most important nutrients for a healthy skeletal system.
The break down of the cartilage leads to pain and stiffness in the joint.
A sprain is an injury to a ligament.
A fracture is a break or crack in a bone.
Review Questions
1. What are the main organs of the skeletal system?
2. Name one tissue of the skeletal system.
3. List four functions of the skeletal system.
4. Name three types of movable joints.
5. Name two things you can do to keep your skeletal system healthy.
Apply Concepts
6. “All joints in the body are movable.” Do you agree with this statement? Explain why or why not.
7. How are the joints in your body similar to levers?
8. Why is calcium important for a healthy skeletal system?
9. The recommended daily amount of calcium for teenagers is 1300 mg. If a person gets only 1000 mg a day, what
percentage of the recommended daily amount are they getting?
10. What part of the skeletal system does osteoarthritis affect?
11. Why might a doctor need to insert pins into a broken bone?
Critical Thinking
12. You are a doctor. An athlete comes to you with a torn ACL and asks you to give him a cast. Tell him why that is
not the correct treatment for his injury.
Further Reading / Supplemental Links
Points to Consider
Next we discuss the muscular system.
• How do you think your skeletal system interacts with your muscular system?
• How could a broken bone affect the functioning of the muscular system?
The Muscular System
Lesson Objectives
Identify the three muscle types in the body.
Describe how skeletal muscles and bones work together to move the body.
Describe how exercise affects the muscular system.
Identify two types of injuries to the muscular system.
Check Your Understanding
• What is muscle tissue?
• What is the function of the muscular system?
aerobic exercises Types of exercises that cause the heart to beat faster and allow the muscles to obtain energy to
contract by using oxygen.
anaerobic exercises Types of exercises that involve short bursts of high-intensity activity; forces the muscles to
obtain energy to contract without using oxygen.
cardiac muscle An involuntary and specialized kind of muscle found only in the heart.
contraction Shortening of muscle fibers.
extensor The muscle that contracts to cause a joint to straighten.
flexor The muscle that contracts to cause a joint to bend.
involuntary muscle A muscle that a person cannot consciously control; cardiac muscle and smooth muscle are
muscle fibers Long, thin cells that can contract; also called muscle cells.
muscular system The body system that allows movement.
physical fitness The ability of your body to carry out your daily activities without getting out of breath, sore, or
overly tired.
skeletal muscle The muscle that is usually attached to the skeleton.
smooth muscle Involuntary muscle found within the walls of organs and structures such as the esophagus, stomach, intestines, and blood vessels.
strain An injury to a muscle in which the muscle fibers tear because the muscle contracts too much or contracts
before the muscle is warmed up.
tendon A tough band of connective tissue that connects a muscle to a bone.
voluntary muscle A muscle that a person can consciously control; skeletal muscle is voluntary.
Types of Muscles
The muscular system is the body system that allows us to move. You depend on many muscles to keep you alive.
Your heart, which is mostly muscle, pumps blood around your body. Muscles are always moving in your body.
Each muscle in the body is made up of cells called muscle fibers. Muscle fibers are long, thin cells that can do
something that other cells cannot do — they are able to get shorter. Shortening of muscle fibers is called contraction.
Nearly all movement in the body is the result of muscle contraction.
Certain muscle movements happen without you thinking about them, while you can control other muscle movements.
Muscles that you can control are called voluntary muscles. Muscles that you cannot control are called involuntary
There are three different types of muscles in the body (Figure 16.21 ):
a. Skeletal muscle is made up of voluntary muscles, usually attached to the skeleton. Skeletal muscles move the
body. They can also contract involuntarily by reflexes. For example, you can choose to move your arm, but
your arm would move automatically if you were to burn your finger on a stove top.
b. Smooth muscle is composed of involuntary muscles found within the walls of organs and structures such as
the esophagus, stomach, intestines, and blood vessels. Unlike skeletal muscle, smooth muscle can never be
under your control.
c. Cardiac muscle is also an involuntary muscle, found only in the heart.
Muscles, Bones, and Movement
Skeletal muscles are attached to the skeleton by tendons. A tendon is a tough band of connective tissue that connects
a muscle to a bone. Tendons are similar to ligaments, except that ligaments join bones to each other.
Muscles move the body by contracting against the skeleton. When muscles contract, they get shorter. When they
relax, they get longer. By contracting and relaxing, muscles pull on bones and allow the body to move. Muscles
work together in pairs. Each muscle in the pair works against the other to move bones at the joints of the body. The
muscle that contracts to cause a joint to bend is called the flexor. The muscle that contracts to cause the joint to
straighten is called the extensor.
For example, the biceps and triceps muscles work together to allow you to bend and straighten your elbow. Your
biceps muscle, shown in Figure 16.22 , contracts, and at the same time the triceps muscle relaxes. The biceps is the
flexor and the triceps is the extensor of your elbow joint. In this way the joints of your body act like levers. This
lever action of your joints decreases the amount of energy you have to spend to make large body movements.
FIGURE 16.21
There are three types of muscles in the
body cardiac skeletal and smooth. Everyone has the same three types of
muscle tissue no matter their age.
FIGURE 16.22
The biceps and triceps act against one
another to bend and straighten the elbow joint. To bend the elbow the biceps contract and the triceps relax. To
straighten the elbow the triceps contract
and the biceps relax.
Muscles and the Nervous System
Muscles are controlled by the nervous system (see the Controlling the Body chapter). Nerves send messages to the
muscular system from the brain. Nerves also send messages to the brain from the muscles. For example, when you
want to move your foot, electrical messages called impulses move along nerve cells from your brain to the muscles
of your foot. At the point at which the nerve cell and muscle cells meet, the electrical message is converted to
a chemical message. The muscle cells receive the chemical message, which causes tiny protein fibers inside the
muscle cells to get shorter. The muscles contract, pulling on the bones, and your foot moves.
Muscles and Exercise
Your muscles are important for carrying out everyday activities. The ability of your body to carry out your daily
activities without getting out of breath, sore, or overly tired is called physical fitness. Physical exercise is any activity
that maintains or improves physical fitness and overall health. Regular physical exercise is important in preventing
lifestyle diseases such as heart disease, cardiovascular disease, Type 2 diabetes, and obesity. Regular exercise
improves the health of the muscular system. Muscles that are exercised are bigger and stronger than muscles that
are not exercised.
Exercise improves both muscular strength and muscular endurance. Muscular strength is the ability of a muscle to
use force during a contraction. Muscular endurance is the ability of a muscle to continue to contract over a long time
without getting tired.
Exercises are grouped into three types depending on the effect they have on the body:
• Aerobic exercises, such as cycling, walking, and running, increase muscular endurance.
• Anaerobic exercises, such as weight training or sprinting, increase muscle strength.
• Flexibility exercises, such as stretching, improve the range of motion of muscles and joints. Regular stretching
helps avoid activity-related injuries.
Anaerobic Exercise and Muscular Strength
Anaerobic exercises cause muscles to get bigger and stronger. Anaerobic exercises use a resistance against which
the muscle has to work to lift or push away. The resistance can be a weight or a person’s own body weight, as shown
in Figure 16.23 . After many muscle contractions, muscle fibers build up larger energy stores and the muscle tissue
gets bigger. The larger a muscle is, the greater the force it can apply to lift a weight or move a body joint. The
muscles of weightlifters are large and strong.
Aerobic Exercise and Muscular Endurance
Aerobic exercises are exercises that cause your heart to beat faster and allow your muscles to use oxygen to contract.
Aerobic exercise causes many different changes in skeletal muscle. Muscle energy stores are increased and the
ability to use oxygen improves. If you exercise aerobically, overtime, your muscles will not get easily tired and you
will use oxygen and food more efficiently. Aerobic exercise also helps improve cardiac muscle. Overtime, the heart
muscles will increase in size and be able to pump a larger volume of blood to your cells. Examples of an aerobic
exercise are shown in Figure 16.24 .
FIGURE 16.23
Anaerobic exercises involve the muscles working against resistance. In
this case the resistance is the person&#8217 s own body weight.
FIGURE 16.24
When done regularly aerobic activities such as cycling make the heart
Muscle Injuries
Sometimes muscles and tendons get injured when a person starts doing an activity before they have warmed up
properly. A warm up is a slow increase in the intensity of a physical activity that prepares muscles for an activity.
Warming up increases the blood flow to the muscles and increases the heart rate. Warmed-up muscles and tendons
are less likely to get injured. For example, before running or playing soccer, a person might jog slowly to warm
muscles and increase their heart rate. Even elite athletes need to warm up, as shown in Figure 16.25 .
A strain happens when muscle fibers tear because the muscle contracts too much or contracts before the muscle is
warmed up. Strains are also known as "pulled muscles." Some injuries are caused by overuse. An overuse injury
happens if the muscle or joint is not rested enough between activities. Overuse injuries often involve tendons.
Overuse of tendons can cause tiny tears within the protein fibers of the tendon. These tiny tears lead to swelling,
pain, and stiffness, a condition called tendinitis. Tendinitis can affect any tendon that is overused. Strains and
tendinitis are usually treated with rest, cold compresses, and stretching exercises that a physical therapist designs for
each patient.
FIGURE 16.25
Warming up before the game helps the
players avoid injuries. Some warm-ups
may include stretching exercises. Some
researchers believe stretching before
activities may help prevent injury.
Proper rest and recovery are also as important to health as exercise is. If you do not get enough rest, your body will
become injured and will not improve or react well to exercise. You can also rest by doing a different activity. For
example, if you run, you can rest your running muscles and joints by swimming. This type of rest is called "active
Anabolic steroids are hormones that cause the body to build up more protein in its cells. Muscle cells, which contain
a lot of protein, get bigger when exposed to anabolic steroids. Your body naturally makes small amounts of anabolic
steroids. They help your body repair from injury, and help to build bones and muscles. Anabolic steroids are used
as medicines to treat people that have illnesses that affect muscle and bone growth. But some athletes who do not
need steroids take them to increase their muscle size. When taken in this way, anabolic steroids can have long-term
affects other body systems. They can damage the person’s kidneys, heart, liver, and reproductive system. If taken by
adolescents, anabolic steroids can cause bones to stop growing, resulting in stunted growth.
Lesson Summary
The body has three types of muscle tissue: skeletal, smooth, and cardiac.
Muscles move the body by contracting against the skeleton.
Muscles are controlled by the nervous system.
Regular exercise improves the health of the muscular system and makes muscles bigger and stronger.
Muscular strength is the ability of a muscle to exert force during a contraction.
Muscular endurance is the ability of a muscle to continue to contract over a long time without getting tired.
A strain is an injury to a muscle in which the muscle fibers tear because the muscle contracts too much or
contracts before the muscle is warmed up.
• Tiny tears and swelling in a tendon results in tendinitis.
Review Questions
1. Name the three types of muscle tissue in the body.
2. Which of the three types of muscles in the body is voluntary?
3. What is a tendon?
4. What is a muscle strain?
Apply Concepts
5. Describe how skeletal muscles and bones work together to move the body.
6. How does aerobic exercise affect the heart?
7. How does aerobic exercise affect skeletal muscle?
8. How does anaerobic exercise affect skeletal muscle?
9. Why is warming up before exercise a good idea?
Critical Thinking
10. A friend of yours says that taking steroids is not bad for their health because humans produce steroids in their
body anyway. Can you explain to them why taking anabolic steroids is dangerous?
Further Reading / Supplemental Links
Points to Consider
Next we move on to the digestive system.
• How does your muscular system depend on your digestive system?
• How does what you choose to eat affect your muscular system and your skeletal system?
MS Food and the Digestive
The above image shows a close-up of the inside of a fleshy tunnel. Could it be an intestine? It could. Or it could be
something else. But what is an intestine? It is an organ in the digestive system. The digestive system does what you
think it does - it digests your food.
But does the inside of your intestine really look like a tunnel? Some would say an intestine looks more like a
mountain range, with peaks and valleys. Why? The peaks and valleys would represent the villi that are inside of the
intestines. They increase the surface area of the intestine so they can absorb as many nutrients as possible.
So what is another function of the digestive system? Absorption. What happens when you get sick? Does the
digestive system work properly? What nutrients and vitamins do you need in order to make sure that your digestive
system and your whole body works properly? What do the villi in the intestines have to do with these processes?
As you read, think about the organs that help you digest and absorb the nutrients in your foods. Also, consider how
healthy eating leads to healthy organ systems.
Food and Nutrients
Lesson Objectives
Explain why the body needs food.
Identify the roles of carbohydrates, proteins, and lipids.
Give examples of vitamins and minerals, and state their functions.
Explain why water is a nutrient.
Check Your Understanding
• What are the four types of organic compounds?
• What do all cells need in order to function?
• What are muscles made of?
calorie Unit used to measure the energy in food.
essential amino acids Amino acids that must come from the proteins in foods; you cannot make these amino
minerals Chemical elements that are needed for body processes.
nutrients Chemicals in food that your body needs.
starch Large, complex carbohydrate; found in foods such as vegetables and grains; broken down by the body into
sugars that provide energy.
trans fat Manufactured fat that is added to certain foods to keep them fresher for longer.
vitamins Substances that the body needs in small amounts to function properly.
Why We Need Food
Did you ever hear the old saying “An apple a day keeps the doctor away”? Do apples really prevent you from getting
sick? Probably not, but eating apples and other fresh fruits can help keep you healthy. The girl shown in Figure 17.1
This girl is eating a salad of vegetables
and leafy green vegetables. Fresh vegetables such as these are excellent food
choices for good health.
is eating fresh vegetables as part of a healthy meal. Why do you need foods like these for good health? What role
does food play in the body?
Your body needs food for three reasons:
a. Food gives your body energy. You need energy for everything you do.
b. Food provides building materials for your body. Your body needs building materials so it can grow and repair
c. Food contains substances that help control body processes. Your body processes must be kept in balance for
good health.
For all these reasons, you must have a regular supply of nutrients. Nutrients are chemicals in food that your body
needs. There are six types of nutrients:
Carbohydrates, proteins, and lipids give your body energy. Proteins provide building materials. Proteins, vitamins,
and minerals help control body processes.
Nutrients that Provide Energy
Molecules of carbohydrates, proteins, and lipids contain energy. When your body digests food, it breaks down the
molecules of these nutrients. This releases the energy so your body can use it. The energy in food is measured in
units called calories.
Carbohydrates are nutrients that include sugars, starches, and fiber. How many grams of carbohydrates you need
each day are shown in Figure 17.2 . It also shows some foods that are good sources of carbohydrates.
Up to the age of 13 years you need
about 130 grams of carbohydrates a
day. Most of the carbohydrates should
be complex. They are broken down
by the body more slowly than simple carbohydrates. Therefore they provide energy longer and more steadily.
What other foods do you think are good
sources of complex carbohydrates
There are two types of carbohydrates: simple and complex.
Simple Carbohydrates
Sugars are small, simple carbohydrates that are found in foods such as fruits and milk. The sugar found in fruits
is called fructose. The sugar found in milk is called lactose. These sugars are broken down by the body to form
glucose, the simplest sugar of all. Glucose is used by cells for energy.
Remember the discussion of cellular respiration in the Cell Functions chapter? Cellular respiration turns glucose
into the usable form of chemical energy, ATP. One gram of sugar provides your body with four Calories of energy.
Some people cannot digest lactose, the sugar in milk. This condition is called lactose intolerance. If people with this
condition drink milk, they may have cramping, bloating, and gas. To avoid these symptoms, they should not drink
milk, or else they should drink special, lactose-free milk.
Complex Carbohydrates
Starch is a large, complex carbohydrate. Starches are found in foods such as vegetables and grains. Starches are
broken down by the body into sugars that provide energy. Like sugar, one gram of starch provides your body with
four calories of energy.
Fiber is another type of large, complex carbohydrate. Unlike sugars and starches, fiber does not provide energy.
However, it has other important roles in the body. There are two types of fiber found in food: soluble fiber and
insoluble fiber. Each type has a different role. Soluble fiber dissolves in water. It helps keep sugar and fat at normal
levels in the blood. Insoluble fiber does not dissolve in water. As it moves through the large intestine, it absorbs
water. This helps keep food waste moist so it can pass easily out of the body.
Eating foods high in fiber helps fill you up without providing too many calories. Most fruits and vegetables are high
in fiber. Some examples are shown in Figure 17.3 .
Proteins are nutrients made up of smaller molecules called amino acids. As discussed in the Introduction to Living
Things chapter, the amino acids are arranged like "beads on a string." These amino acid chains then fold up into a
three-dimensional molecule. Proteins have several important roles in the body. For example, proteins:
Make up muscles.
Help control body processes.
Help the body fight off bacteria and other “foreign invaders.”
Carry substances in the blood.
If you eat more proteins than you need for these purposes, the extra proteins are used for energy. One gram of
protein provides four calories of energy. This is the same amount as one gram of sugar or starch. How many grams
of proteins you need each day are shown in Figure 17.4 . It also shows some foods that are good sources of proteins.
There are many different amino acids, the building blocks of proteins, but your body needs only 20 of them. Your
body can make ten of these amino acids from simpler substances. The other ten amino acids must come from the
proteins in foods. These ten are called essential amino acids. Only animal foods, such as milk and meat, contain
all ten essential amino acids in a single food. Plant foods are missing one or more essential amino acids. However,
by eating a combination of plant foods, such as beans and rice, you can get all ten essential amino acids.
Lipids are nutrients such as fats that store energy. The heart and skeletal muscles rely mainly on lipids for energy.
One gram of lipids provides nine calories of energy. This is more than twice the amount provided by carbohydrates
or proteins. Lipids have several other roles in the body. For example, lipids:
Protect nerves.
Help control blood pressure.
Help blood to clot.
Make up the membranes that surround cells.
Between the ages of 9 and 13 years girls need about 26 grams of fiber a day and boys need about 31 grams of
fiber a day. Do you know other foods that are high in fiber
Between the ages of 9 and 13 years you need about 34 grams of proteins a day. What other foods do you think
are good sources of proteins
Fats are one type of lipid. Stored fat gives your body energy to use for later. It’s like having money in a savings
account. It’s there in case you need it. Stored fat also cushions and protects internal organs. In addition, it insulates
the body. It helps keep you warm in cold weather.
Fats and other lipids are necessary for life. However, they can be harmful if you eat too much of them, or the wrong
type of fats. Fats can build up in the blood and damage blood vessels. This increases the risk of heart disease.
There are two types of lipids, saturated and unsaturated.
a. Saturated lipids can be unhealthy, even in very small amounts. They are found mainly in animal foods, such
as meats, whole milk, and eggs. Saturated lipids increase cholesterol levels in the blood. Cholesterol is a fatty
substance that is found naturally in the body. Too much cholesterol in the blood can lead to heart disease. It is
best to limit the amount of saturated lipids in your diet.
a. Unsaturated lipids are found mainly in plant foods, such as vegetable oil, olive oil, and nuts. Unsaturated
lipids are also found in fish, such as salmon. Unsaturated lipids are needed in small amounts for good health
because your body cannot make them. Most lipids in your diet should be unsaturated.
Another type of lipid is called trans fat. Trans fats are manufactured and added to certain foods to keep them fresher
for longer. Foods that contain trans fats include cakes, cookies, fried foods, and margarine. Eating foods that contain
trans fats increases the risk of heart disease. You should do your best to eat fewer foods that contain it.
Beginning in 2010, California banned trans fats from restaurant products, and, beginning in 2011, from all retail
baked goods.
Vitamins and Minerals
Vitamins and minerals are also nutrients. They do not provide energy, but they are needed for good health.
Vitamins are substances that the body needs in small amounts to function properly. Humans need 13 different
vitamins. Some of them are listed in Table 17.1 . The table also shows how much of each vitamin you need each
day. Vitamins have many roles in the body. For example, Vitamin A helps maintain good vision. Vitamin B9 helps
form red blood cells. Vitamin K is needed for blood to clot when you have a cut or other wound.
TABLE 17.1: Vitamins Needed For Good Health
One Reason You Need It
Some Foods that Have It
Vitamin A
Needed for good vision
Vitamin B1
Needed for healthy nerves
Vitamin B3
Needed for healthy skin
and nerves
Needed to make red blood
Needed for healthy nerves
Carrots, spinach, milk,
Whole wheat, peas, meat,
beans, fish, peanuts
Beets, liver, pork, turkey,
fish, peanuts
Liver, peas, dried beans,
green leafy vegetables
Meat, liver, milk, shellfish, eggs
Oranges, grapefruits, red
peppers, broccoli
Milk, salmon, tuna, eggs
Vitamin B9
Vitamin B12
Vitamin C
Vitamin D
Vitamin K
Needed for growth and repair of tissues
Needed for healthy bones
and teeth
Needed for blood to clot
Spinach, Brussels sprouts,
milk, eggs
How Much of It You Need
Each Day (at ages 9–13
600 µg (1 µg = 1 x 10 6 g)
0.9 mg (1 mg = 1 x 10
12 mg
300 µg
1.8 µg
45 mg
5 µg
60 µg
Some vitamins are produced in the body. For example, vitamin D is made in the skin when it is exposed to sunlight.
Vitamins B12 and K are produced by bacteria that normally live inside the body. Most other vitamins must come
from foods. Foods that are good sources of vitamins are listed in Table 17.1 . They include whole grains, vegetables,
fruits, and milk.
Not getting enough vitamins can cause health problems. For example, too little vitamin C causes a disease called
scurvy. People with scurvy have bleeding gums, nosebleeds, and other symptoms. Getting too much of some
vitamins can also cause health problems. The vitamins to watch out for are vitamins A, D, E, and K. These vitamins
are stored by the body, so they can build up to high levels. Very high levels of these vitamins can even cause death,
although this is very rare.
Minerals are chemical elements that are needed for body processes. Minerals are different from vitamins because
they do not contain the element carbon. Minerals that you need in relatively large amounts are listed in Table 17.2 .
Minerals that you need in smaller amounts include iodine, iron, and zinc.
Minerals have many important roles in the body. For example, calcium and phosphorus are needed for strong bones
and teeth. Potassium and sodium are needed for muscles and nerves to work normally.
TABLE 17.2: Minerals Needed For Good Health
One Reason You Need It
Some Foods that Have It
Needed for strong bones
and teeth
Needed for proper balance
of water and salts in body
Needed for strong bones
Milk, soy milk, green
leafy vegetables
Table salt, most packaged
Whole grains, green leafy
vegetables, nuts
Meat, poultry, whole
Meats, grains, bananas,
orange juice
Table salt, most packaged
Needed for strong bones
and teeth
Needed for muscles and
nerves to work normally
Needed for muscles and
nerves to work normally
How Much of It You Need
Each Day (at ages 9–13
1,300 mg
2.3 g
240 mg
1,250 mg
4.5 g
1.5 g
Your body cannot produce any of the minerals that it needs. Instead, you must get minerals from the foods you eat.
Good sources of minerals are listed in Table 17.2 . They include milk, green leafy vegetables, and whole grains.
Not getting enough minerals can cause health problems. For example, too little calcium may cause osteoporosis.
This is a disease in which bones become soft and break easily. Getting too much of some minerals can also cause
health problems. Many people get too much sodium. Sodium is added to most packaged foods. People often add
more sodium to their food by using table salt. Too much sodium causes high blood pressure in some people.
Did you know that water is also a nutrient? By weight, your cells are about two-thirds water, so you cannot live
without it. In fact, you can survive for only a few days without water.
You lose water in each breath you exhale. You also lose water in sweat and urine. If you do not take in enough water
to replace the water that you lose, you may develop dehydration. Symptoms of dehydration include dry mouth,
headaches, and feeling dizzy. Dehydration can be very serious. Severe dehydration can even cause death.
When you exercise, especially on a hot day, you lose more water in sweat than you usually do. You need to drink
extra water before, during, and after exercise. The children in Figure 17.5 are drinking water while playing outside
on a warm day. They need to drink water to avoid dehydration.
Getting too much water can also be dangerous. Excessive water may cause a condition called hyponatremia. In this
condition, water collects in the brain and causes it to swell. Hyponatremia can cause death. It requires emergency
medical care.
Lesson Summary
The body needs food for energy, building materials, and substances that help control body processes.
Carbohydrates, proteins, and lipids provide energy and have other important roles in the body.
Vitamins and minerals do not provide energy but are needed in small amounts for the body to function properly.
The body must have water to survive.
When you are active outside on a warm
day it&#8217 s important to drink plenty
of water. You need to replace the water
you lose in sweat.
Review Questions
1. What are three reasons that your body needs food?
2. Which nutrients can be used for energy?
3. What are some foods that are good sources of vitamin C?
4. What are two minerals that are needed for strong bones and teeth?
Apply Concepts
5. Name two types of fiber and state the role of each type of fiber in the body.
6. Your body needs 20 different amino acids. Why do you need to get only ten of these amino acids from food?
Name foods you can eat to get these ten amino acids.
7. Compare and contrast saturated and unsaturated lipids.
8. Identify three vitamins that are produced in the body. How are they produced?
9. Why do you need to drink extra water when you exercise on a hot day? What might happen if you did not drink
extra water?
Critical Thinking
10. List some of the functions of proteins in the body. Based on your list, predict health problems people might have
if they do not get enough proteins in foods.
Further Reading / Supplemental Links
• Alexandra Powe Allred. Nutrition. Perfection Learning, 2005.
• Ann Douglas and Julie Douglas. Body Talk: The Straight Facts on Fitness, Nutrition, and Feeling Great about
Yourself! Maple Tree Press, 2006.
• DK Publishing. Food. DK Children, 2005.
• Donna Shryer. Body Fuel: A Guide to Good Nutrition. Marshall Cavendish Children’s Books, 2007.
• Linda Bickerstaff. Nutrition Sense. Rosen Central, 2008.
CK–12. High School Biology. Chapter 38, Lesson 1.
Points to Consider
Think about how you can be sure you are getting enough nutrients.
• Do you think knowing the nutrients in the foods you eat are important?
• Do you have to keep track of all the nutrients you eat, or is there an easier way to choose foods that provide
the nutrients you need?
Choosing Healthy Foods
Lesson Objectives
• State how to use MyPyramid to get the proper balance of nutrients.
• Describe how to read food labels to choose foods wisely.
• Explain how to balance food with exercise.
Check Your Understanding
• What is a nutrient?
• Why do you need extra energy when you exercise?
ingredient A specific item that a food contains.
main ingredient The ingredient that is present in the food in the greatest amount.
MyPlate Visual representation of the relative daily portions of various food groups; replaced MyPyramid in 2011.
MyPyramid Diagram that shows how much you should eat each day of foods from six different food groups.
nutrition facts label The label on packaged food that shows the nutrients in the food.
obesity Having a very high percentage of body fat; obese people are at least 20 percent heavier than their healthy
weight range.
serving size Tells you how much of the food you should eat to get the nutrients listed on the label.
Foods such as whole grain breads, fresh fruits, and fish provide nutrients you need for good health. But different
foods give you different types of nutrients. You also need different amounts of each nutrient. How can you choose the
right mix of foods to get the proper balance of nutrients? Two tools can help you choose foods wisely: MyPyramid
and food labels.
MyPyramid is a diagram that shows how much you should eat each day of foods from six different food groups. It
recommends the amount of nutrients you need based on your age, your sex, and your level of activity. MyPyramid
is shown in Figure 17.6 . The six food groups in MyPyramid are:
Grains, such as bread, rice, pasta, and cereal.
Vegetables, such as spinach, broccoli, carrots, and sweet potatoes.
Fruits, such as oranges, apples, bananas, and strawberries.
Oils, such as vegetable oil, canola oil, olive oil, and peanut oil.
Dairy, such as milk, yogurt, cottage cheese, and other cheeses.
Meat and beans, such as chicken, fish, soybeans, and kidney beans.
MyPyramid can help you choose foods
wisely for good health. Each colored
band represents a different food group.
The key shows which food group each
color represents. Which colored band of
MyPyramid is widest Which food group
does it represent
Using MyPyramid
In MyPyramid, each food group is represented by a band of a different color. For example, grains are represented
by an orange band, and vegetables are represented by a green band. The wider the band, the more foods you should
choose from that food group each day.
The orange band in MyPyramid is the widest band. This means that you should choose more foods from the grain
group than from any other single food group. The green, blue, and red bands are also relatively wide. Therefore,
you should choose plenty of foods from the vegetable, dairy, and fruit groups as well. You should choose the fewest
foods from the food group with the narrowest band. Which band is narrowest? Which food group does it represent?
Healthy Eating Guidelines
Did you ever hear the saying, “variety is the spice of life”? Variety is also the basis of a healthy eating plan. When
you choose foods based on MyPyramid, you should choose a variety of different foods. Follow these guidelines
to make the wisest food choices for good health. Keep in mind that nutritional rules may change as you get older.
As food provides energy and nutrients for growth and development, nutritional requirements may vary with body
weight, age, sex, activity, and body functioning.
• Make at least half your daily grain choices whole grains. Examples of whole grains are whole wheat bread,
whole wheat pasta, and brown rice.
• Choose a variety of different vegetables each day. Be sure to include both dark green vegetables, such as
spinach and broccoli, and orange vegetables, such as carrots and sweet potatoes.
• Choose a variety of different fruits each day. Select mainly fresh fruits rather than canned fruits and whole
fruits instead of fruit juices.
• When choosing oils, go for unsaturated oils, such as olive oil, canola oil, or vegetable oil.
• Choose low-fat or fat-free milk and other dairy products. For example, select fat-free yogurt and low-fat
• For meats, choose fish, chicken, and lean cuts of beef. Also, be sure to include beans, nuts, and seeds.
What about Ice Cream, Cookies, and Potato Chips?
Are you wondering where foods like ice cream, cookies, and potato chips fit into MyPyramid? The white tip of
MyPyramid represents foods such as these. These are foods that should be eaten only in very small amounts and not
very often. Such foods contain very few nutrients, and are called nutrient-poor. Instead, they are high in fats, sugars,
and sodium, which are nutrients that you should limit in a healthy eating plan. Ice cream, cookies, and potato chips
are also high in calories. Eating too much of them may lead to unhealthy weight gain.
In June 2011, the United States Department of Agriculture replaced My Pyramid with MyPlate. MyPlate depicts
the relative daily portions of various food groups. See for further information.
The following guidelines accompany MyPlate:
1. Balancing Calories
• Enjoy your food, but eat less.
• Avoid oversized portions.
2. Foods to Increase
• Make half your plate fruits and vegetables.
• Make at least half your grains whole grains.
• Switch to fat-free or low-fat (1%) milk.
3. Foods to Reduce
• Compare sodium in foods like soup, bread, and frozen meals [U+2015] and choose the foods with lower
• Drink water instead of sugary drinks.
MyPlate is a visual guideline for balanced eating replacing MyPyramid in
Food Labels
In the United States, packaged foods are required by law to have nutrition facts labels. A nutrition facts label shows
the nutrients in a food. Packaged foods are also required to list their ingredients. An ingredient is a specific item
that a food contains.
Using Nutrition Facts Labels
An example of a nutrition facts label is shown in Figure 17.8 . The information listed at the right of the label tells
you what to look for. At the top of the label, look for the serving size. The serving size tells you how much of the
food you should eat to get the nutrients listed on the label. A cup of food from the label in Figure 17.8 is a serving.
The calories in one serving are listed next. In this food, there are 250 calories per serving.
Next on the nutrition facts label, look for the percent daily values (% DV) of nutrients. Remember the following tips
when reading a food label:
A food is low in a nutrient if the percent daily value of the nutrient is 5% or less.
The healthiest foods are low in nutrients such as fats and sodium.
A food is high in a nutrient if the percent daily value of the nutrient is 20% or more.
The healthiest foods are high in nutrients such as fiber and proteins.
Look at the percent daily values on the food label in Figure 17.8 . Which nutrients have values of 5% or less? These
are the nutrients that are low in this food. They include fiber, vitamin A, vitamin C, and iron. Which nutrients have
values of 20% or more? These are the nutrients that are high in this food. They include sodium, potassium, and
Reading nutrition facts labels can help
you choose healthy foods. Look at the
nutrition facts label shown here. Do you
think this food is a good choice for a
healthy eating plan Why or why not
Using Ingredients Lists
The food label in Figure 17.9 includes the list of ingredients in a different food. The ingredients on food labels are
always listed from the highest amount to the lowest amount. This means that the main ingredient is listed first. The
main ingredient is the ingredient that is present in the food in the greatest amount. As you go down the list, the
ingredients are present in smaller and smaller amounts.
This food label includes the list of ingredients in the food. The main ingredient is enriched wheat flour followed
by high-fructose corn syrup. Why should
you avoid foods with ingredients such as
these at the top of the ingredients list
Reading the ingredients lists on food labels can help you choose the healthiest foods. At the top of the list, look
for ingredients such as whole grains, vegetables, milk, and fruits. These are the ingredients you need in the greatest
amounts for balanced eating. Avoid foods that list fats, oils, sugar, or salt at the top of the list. For good health, you
should avoid getting too much of these ingredients. Be aware that ingredients such as corn syrup are sugars.
You should also use moderation when eating foods that contain ingredients such as white flour or white rice. These
ingredients have been processed, and processing removes nutrients. The word "enriched" is a clue that an ingredient
has been processed. Ingredients are enriched with added nutrients to replace those lost during processing. However,
enriched ingredients are still likely to have fewer nutrients than unprocessed ingredients.
Balancing Food with Exercise
Look at MyPyramid in Figure 17.6 . Note the person walking up the side of the pyramid. This shows that exercise
is important for balanced eating. Exercise helps you use any extra energy in the foods you eat. The more active you
are, the more energy you use. You should try to get at least an hour of physical activity just about every day. Figure
17.10 shows some activities that can help you use extra energy.
FIGURE 17.10
All of these activities are good ways to exercise and use extra energy. The calories given for each activity are the
number of calories used in an hour by a person that weighs 100 pounds. Which of these activities uses the most
calories Which of the activities do you enjoy
How Does Fat Form?
Any unused energy in food is stored in the body as fat. This is true whether the extra energy comes from carbohydrates, proteins, or lipids. What happens if you take in more energy than you use, day after day? You will store more
and more fat and become overweight.
Eventually, you may become obese. Obesity is having a very high percentage of body fat. Obese people are at least
20 percent heavier than their healthy weight range. The excess body fat of obesity is linked to many diseases. Obese
people often have serious health problems, such as diabetes, high blood pressure, and high cholesterol. They are also
more likely to develop arthritis and some types of cancer. People that remain obese during their entire adulthood
usually do not live as long as people that stay within a healthy weight range.
The current generation of children and teens is the first generation in our history that may have a shorter life than
their parents. The reason is their high rate of obesity and the health problems associated with obesity. You can avoid
gaining weight and becoming obese. The choice is yours. Choose healthy foods by using MyPyramid and reading
food labels. Then get plenty of exercise to balance the energy in the foods you eat.
Lesson Summary
• MyPyramid shows how much you should eat each day of foods from six different food groups.
• Reading food labels can help you choose the healthiest foods.
• Regular exercise helps you use extra energy and avoid unhealthy weight gain.
Review Questions
1. List the six food groups represented by MyPyramid.
2. Which food group contains soybeans, kidney beans, and fish?
3. What guideline should you follow in choosing foods from the grains food group?
4. Which ingredient is always listed first on a food label?
5. What happens if you take in more energy than you use, day after day?
Apply Concepts
6. Explain how you can use MyPyramid to choose foods that provide the proper balance of nutrients.
7. Why should you limit foods like ice cream and potato chips in a healthy eating plan?
8. Explain how you can use food labels to choose foods that are high in fiber.
9. Why should you try to avoid foods with processed ingredients? What are some examples of processed ingredients?
Critical Thinking
10. You are trying to convince your friends that it is worth it to eat healthy and do physical activity. What will you
tell them? Give examples from the chapter.
Further Reading / Supplemental Links
• Eric Schlosser and Charles Wilson. Chew on This: Everything You Don’t Want to Know about Fast Food.
Houghton Mifflin, 2006.
• John Burstein. The Shape of Good Nutrition: The Food Pyramid. Crabtree Publishing Company, 2008.
• Rose McCarthy. Food Labels: Using Nutrition Information to Create a Healthy Diet. Rosen Publishing
Group, 2008.
• Sandra Giddens. Making Smart Choices about Food, Nutrition, and Lifestyle. Rosen Central, 2008.
CK–12. High School Biology. Chapter 38, Lesson 1.
• acrobat/nutfacts.pdf dms/foodlab.html
Points to Consider
Next we discuss the digestive system.
• Discuss how you think foods may be broken down into nutrients that your body can use? For example, how
do you think an apple becomes simple sugars that your body can use for energy? Or how might a piece of
cheese become proteins that your body can use for building materials?
The Digestive System
Lesson Objectives
List the functions of the digestive system.
Explain the role of enzymes in digestion.
Describe the digestive organs and their functions.
Explain the roles of helpful bacteria in the digestive system.
List ways to help keep your digestive system healthy.
Check Your Understanding
• What is a chemical reaction?
• What is an enzyme?
• What are bacteria?
absorption Process in which substances are taken up by the blood; after food is broken down into small nutrient
molecules, the molecules are absorbed by the blood.
chemical digestion Digestion in which large food molecules are broken down into small nutrient molecules.
digestion Process of breaking down food into nutrients.
digestive system Body system that breaks down food, absorbs nutrients, and gets rid of solid food waste.
duodenum The first part of the small intestine; where most chemical digestion takes place.
esophagus The narrow tube that carries food from the throat to the stomach.
food allergies A condition in which the immune system reacts to harmless substances in food as though they were
ileum The third part of the small intestine; covered with villi; the few remaining nutrients are absorbed in the
jejunum The second part of the small intestine; where most nutrients are absorbed into the blood; lined with tiny
“fingers” called villi.
large intestine The relatively wide tube between the small intestine and anus where excess water is absorbed from
food waste.
mechanical digestion Digestion in which large chunks of food are broken down into small pieces.
small intestine The narrow tube between the stomach and large intestine where most chemical digestion and
absorption of nutrients take place.
stomach The sac-like organ at the end of the esophagus where proteins are digested.
villi Contain microscopic blood vessels; nutrients are absorbed into the blood through these tiny vessels; located
on the jejunum and the ileum.
What Does the Digestive System Do?
Nutrients in the foods you eat are needed by the cells of your body. How do the nutrients in foods get to your body
cells? What organs and processes break down the foods and make the nutrients available to cells? The organs are
those of the digestive system. The processes are digestion and absorption.
The digestive system is the body system that breaks down food and absorbs nutrients. It also gets rid of solid food
waste. The main organs of the digestive system are shown in Figure 17.11 .
FIGURE 17.11
This drawing shows the major organs of
the digestive system. Trace the path of
food through the organs of the digestive
system as you read about them in this
Digestion is the process of breaking down food into nutrients. There are two types of digestion, mechanical and
chemical. In mechanical digestion, large chunks of food are broken down into small pieces. This is a physical
process. In chemical digestion, large food molecules are broken down into small nutrient molecules. This is a
chemical process.
Absorption is the process that allows substances you eat to be taken up by the blood. After food is broken down into
small nutrient molecules, the molecules are absorbed by the blood. After absorption, the nutrient molecules travel
in the bloodstream to cells throughout the body.
Some substances in food cannot be broken down into nutrients. They remain behind in the digestive system after the
nutrients are absorbed. Any substances in food that cannot be digested and absorbed pass out of the body as solid
waste. The process of passing solid food waste out of the body is called elimination.
The Role of Enzymes in Digestion
Chemical digestion could not take place without the help of digestive enzymes. An enzyme is a protein that speeds
up chemical reactions in the body. Digestive enzymes speed up chemical reactions that break down large food
molecules into small molecules.
Did you ever use a wrench to tighten a bolt? You could tighten a bolt with your fingers, but it would be difficult and
slow. If you use a wrench, you can tighten a bolt much more easily and quickly. Enzymes are like wrenches. They
make it much easier and quicker for chemical reactions to take place. Like a wrench, enzymes can also be used over
and over again. But you need the appropriate size and shape of the wrench to efficiently tighten the bolt, just like
each enzyme is specific for the reaction it helps.
Digestive enzymes are released, or secreted, by the organs of the digestive system. Examples of digestive enzymes
Amylase, produced in the mouth. It helps break down large starches molecules into smaller sugar molecules.
Pepsin, produced in the stomach. Pepsin helps break down proteins into amino acids.
Trypsin, produced in the pancreas. Trypsin also breaks down proteins.
Pancreatic lipase, produced in the pancreas. It is used to break apart fats.
Deoxyribonuclease and ribonuclease, produced in the pancreas. They are enzymes that break bonds in nucleic
acids like DNA and RNA.
Bile salts are bile acids that help to break down fat. Bile acids are made in the liver. When you eat a meal, bile is
secreted into the intestine, where it breaks down the fats. Bile acids also help to remove cholesterol from the body.
Hormones and Digestion
If you are a typical teenager, you like to eat. For your body to break down, absorb and spread the nutrients throughout
your body, your digestive system and endocrine system need to work together. The endocrine system sends hormones
around your body to communicate between cells like chemical messengers.
Digestive hormones are made by cells lining the stomach and small intestine. These hormones cross into the blood
where they can affect other parts of the digestive system. Some of these hormones are listed below.
Gastrin, which signals the secretion of gastric acid.
Cholecystokinin, which signals the secretion of pancreatic enzymes.
Secretin, which signals secretion of water and bicarbonate from the pancreas.
Ghrelin, which signals when you are hungry.
Gastric inhibitory polypeptide, which stops or decreases gastric secretion. It also causes the release of insulin
in response to high blood glucose levels.
Digestive Organs and Their Roles
The mouth and stomach are just two of the organs of the digestive system. Other digestive system organs are the
esophagus, small intestine, and large intestine. From Figure 17.10 you can see that the digestive organs form a long
tube. In adults, this tube is about 30 feet long! At one end of the tube is the mouth. At the other end is the anus.
Food enters the mouth and then passes through the rest of the digestive system. Food waste leaves the body through
the anus.
The organs of the digestive system are lined with muscles. The muscles contract, or tighten, to push food through
the system. This is shown in Figure 17.12 . The muscles contract in waves. The waves pass through the digestive
system like waves through a slinky. This movement of muscle contractions is called peristalsis. Without peristalsis,
food would not be able to move through the digestive system. Peristalsis is an involuntary process, which means that
it occurs without your conscious control.
FIGURE 17.12
This diagram shows how muscles push food through the digestive system. Muscle contractions travel through the
system in waves pushing the food ahead of them. This is called peristalsis.
The liver, gall bladder, and pancreas are also organs of the digestive system. They are shown in Figure 17.13 . Food
does not pass through these three organs. However, these organs are important for digestion. They secrete or store
enzymes or other chemicals that are needed to help digest food chemically.
Mouth, Esophagus, and Stomach
The mouth is the first organ that food enters. But digestion may start even before you put the first bite of food into
your mouth. Just seeing or smelling food can cause the release of saliva and digestive enzymes in your mouth.
Once you start eating, saliva wets the food, which makes it easier to break up and swallow. Digestive enzymes,
including amylase, start breaking down starches into sugars. Your tongue helps mix the food with the saliva and
Your teeth also help digest food. Your front teeth are sharp. They cut and tear food when you bite into it. Your
FIGURE 17.13
This drawing shows the liver gall bladder and pancreas. These organs are
part of the digestive system. Food does
not pass through them but they secrete
substances needed for chemical digestion.
back teeth are broad and flat. They grind food into smaller pieces when you chew. Chewing is part of mechanical
digestion. Your tongue pushes the food to the back of your mouth so you can swallow it. When you swallow, the
lump of chewed food passes down your throat to your esophagus.
The esophagus is a narrow tube that carries food from the throat to the stomach. Food moves through the esophagus
because of peristalsis. At the lower end of the esophagus, a circular muscle controls the opening to the stomach. The
muscle relaxes to let food pass into the stomach. Then the muscle contracts again to prevent food from passing back
into the esophagus.
Some people think that gravity moves food through the esophagus. If that were true, food would move through the
esophagus only when you are sitting or standing upright. In fact, because of peristalsis, food can move through the
esophagus no matter what position you are in — even upside down! Just don’t try to swallow food when you upside
down! You could choke if you try to swallow when you are not upright.
The stomach is a sac-like organ at the end of the esophagus. It has thick muscular walls. The muscles contract and
relax. This moves the food around and helps break it into smaller pieces. Mixing the food around with the enzyme
pepsin and other chemicals helps digest proteins.
Water, salt, and simple sugars can be absorbed into the blood from the stomach. Most other substances are broken
down further in the small intestine before they are absorbed. The stomach stores food until the small intestine is
ready to receive it. A circular muscle controls the opening between the stomach and small intestine. When the small
intestine is empty, the muscle relaxes. This lets food pass from the stomach into the small intestine.
Small Intestine
The small intestine is narrow tube that starts at the stomach and ends at the large intestine (see Figure 17.11 ). In
adults, the small intestine is about 23 feet long. It is made up of three parts, the duodenum, the jejunum and the
a. The duodenum is the first part of the small intestine. This is where most chemical digestion takes place.
Many enzymes and other chemicals are secreted here.
b. The jejunum is the second part of the small intestine. This is where most nutrients are absorbed into the
blood. The jejunum is lined with tiny “fingers” called villi. A magnified picture of villi is shown in Figure
17.14 . Villi contain very tiny blood vessels. Nutrients are absorbed into the blood through these tiny vessels.
There are millions of villi, so altogether there is a very large area for absorption to take place. In fact, villi
make the inner surface area of the small intestine 1,000 times larger than it would be without them. The entire
inner surface area of the small intestine is about as big as a basketball court!
c. The ileum is the third part of the small intestine. Like the jejunum, the ileum is covered with villi. A few
remaining nutrients are absorbed in the ileum. From the ileum, any remaining food waste passes into the large
The small intestine is much longer than the large intestine. So why is it called “small”? If you compare the small and
large intestines in Figure 17.11 , you will see why. The small intestine is smaller in width than the large intestine.
Large Intestine
The large intestine is a wide tube that connects the small intestine with the anus. In adults, it is about 5 feet long.
Waste enters the large intestine from the small intestine in a liquid state. As the waste moves through the large
intestine, excess water is absorbed from it. After the excess water is absorbed, the remaining solid waste is called
Circular muscles control the anus. They relax to let the feces pass out of the body through the anus. After feces pass
out of the body, they are called stool. Releasing the stool from the body is referred to as a bowel movement.
FIGURE 17.14
This is what the villi lining the small intestine look like when magnified. Each
one is actually only about 1 millimeter
long. Villi are just barely visible with the
unaided eye.
The liver has a wide range of functions, a few of which are:
Removing toxins from the blood.
Keeping glucose levels stable.
Creating proteins.
Producing biochemicals for digestion.
The liver is necessary for survival. You cannot live without a liver. The liver is one of the most important organs in
the body when it comes to getting rid of toxins, especially from the gut. The liver filters blood from the intestine.
This filtering process can remove microorganisms such as bacteria, fungi, viruses and parasites from the blood.
Almost 2 quarts of blood pass through the liver every minute. Since the liver also ensures that glucose levels remain
stable, people with liver problems are at risk for diabetes.
Bacteria in the Digestive System
Your large intestine is not just made up of cells. It is also an ecosystem, home to trillions of bacteria. But don’t
worry. Most of these bacteria are helpful. They have several roles in the body. For example, intestinal bacteria:
Produce vitamins B12 and K.
Control the growth of harmful bacteria.
Break down poisons in the large intestine.
Break down some substances in food that cannot be digested, such as fiber and some starches and sugars.
Keeping Your Digestive System Healthy
Most of the time, you probably aren’t aware of your digestive system. It works well without causing any problems.
But most people have problems with their digestive system at least once in awhile. Did you ever eat something that
didn’t “agree” with you? Maybe you had a stomachache or felt sick to your stomach? Maybe you had diarrhea?
These could be symptoms of foodborne illness.
Foodborne Illness
Harmful bacteria can enter your digestive system in food and make you sick. This is called foodborne illness. The
bacteria, or the toxins they produce, may cause vomiting or cramping, in addition to the symptoms mentioned above.
You can help prevent foodborne illness by following a few simple rules.
• Keep hot foods hot and cold foods cold. This helps prevent any bacteria in the foods from multiplying.
• Wash your hands before you prepare or eat food. This helps prevent bacteria on your hands from getting on
the food.
• Wash your hands after you touch raw foods such as meats, poultry, fish, or eggs. These foods often contain
bacteria that your hands could transfer to your mouth.
• Cook meats, poultry, fish, and eggs thoroughly before eating them. The heat of cooking kills any bacteria the
foods may contain, so they cannot make you sick.
Food Allergies
Food allergies are like other allergies. They occur when the immune system reacts to harmless substances as though
they were harmful. Almost 10 percent of children have food allergies. Some of the foods most likely to cause
allergies are shown in Figure 17.15 .
Eating foods you are allergic to may cause vomiting, diarrhea, or skin rashes. Some people are very allergic to certain
foods. Eating even tiny amounts of the foods causes them to have serious symptoms, such as difficulty breathing. If
they eat the foods by accident, they may need emergency medical treatment.
If you think you may have food allergies, a doctor can test you to find out for sure. The tests will identify which
foods you are allergic to. Then you can avoid eating these foods. This is the best way to prevent the symptoms of
food allergies. To avoid the foods you are allergic to, you may have to read food labels carefully. This is especially
likely if you are allergic to common food ingredients, such as soybeans, wheat, or peanuts.
A food intolerance, or food sensitivity, is different from a food allergy. A food intolerance happens when the
digestive system is unable to break down a certain type of food. This can result in stomach cramping, diarrhea,
tiredness, and weight loss. Food intolerances are often mistakenly called allergies. Lactose intolerance is a food
intolerance. A person who is lactose intolerant does not make enough lactase, the enzyme that breaks down the milk
sugar lactose. About 75 percent of the world’s population is lactose intolerant.
Lesson Summary
• The digestive system breaks down food, absorbs nutrients, and gets rid of food wastes.
• Digestive enzymes speed up the reactions of chemical digestion.
• The main organs of the digestive system are the mouth, esophagus, stomach, small intestine, and large intestine.
• Bacteria in the large intestine produce vitamins and have other roles in the body.
FIGURE 17.15
Some of the foods that commonly cause
allergies are shown here. They include
nuts eggs fish milk and shellfish. Are
you allergic to any of these foods
Review Questions
1. What are three functions of the digestive system?
2. Identify two roles of helpful bacteria in the large intestine.
3. List two rules that can help prevent foodborne illness.
Apply Concepts
4. Describe the roles of the mouth in digestion.
5. In which organs of the digestive system does absorption of nutrients take place? |
6. Explain the role of enzymes in digestion. Give examples to illustrate your answer.
7. Describe peristalsis, and explain why it is necessary for digestion.
8. How can the inner surface area of the small intestine be as big as a basketball court? How does this help the small
intestine absorb nutrients?
Critical Thinking
9. Assume a person has an illness that prevents the large intestine from doing its normal job. Why might the person
have diarrhea?
10. Explain why eating high-fiber foods can help prevent constipation.
Further Reading / Supplemental Links
CK–12, High School Biology, Chapter 38, Lesson 2.
• Carol Ballard. The Digestive System. Heinemann Library, 2003.
• Robert J. Sullivan. Digestion and Nutrition. Chelsea House Publications, 2004.
• Sherri Mabry Gordon. Peanut Butter, Milk, and Other Deadly Threats: What You Should Know about Food
Allergies. Enslow Publishers, 2006.
• Steve Parker. Break It Down: The Digestive System. Raintree, 2006.
• dms/a2z-b.html
Points to Consider
• After nutrients are absorbed into the blood, think about how the blood could carry them to all the cells of the
body. How does the blood travel? What keeps the blood moving?
MS Cardiovascular System
18.3 B LOOD
What does the above image show? It could be any of a number of things. A bowl of melted strawberry ice cream.
Some chewed bubble gum. But the above image is actually a close-up of cardiac muscle, the muscle that makes up
your heart. If you recall, cardiac muscle is one of three muscle types in the human body. Is cardiac muscle found
anywhere else in the body? No. Cardiac muscle is only found in the heart!
Why is cardiac muscle only found in the heart? Why does the heart need its own special muscle? What does blood
do, anyway? If a heart pumps blood, how does the heart get the blood it needs to keep pumping? What happens if
the heart does not get enough blood?
You may have heard of a heart attack - but what is actually happening in the heart when that happens?
Consider these questions about the heart and cardiac muscle as you read about one of the most important and
intriguing systems in the body, the cardiovascular system.
18.1 Introduction to the Cardiovascular System
Lesson Objectives
Identify the main structures of the cardiovascular system.
Identify three types of blood vessels.
Describe the differences between the pulmonary and the systemic circulations.
Identify the main structures of the lymphatic system.
Outline how the cardiovascular and the lymphatic systems work together.
Check Your Understanding
• What is an organ system?
• What are the three types of muscles found in the human body?
arteries Blood vessels that carry blood away from the heart.
blood A body fluid that is a type of connective tissue; moves oxygen and other compounds throughout the body.
capillaries The smallest and narrowest blood vessels in the body.
lymphatic system A network of vessels and tissues that carry a clear fluid called lymph; includes lymph nodes,
lymph ducts, and lymph vessels.
plasma The straw-colored fluid in blood; about 90 percent water and about 10 percent dissolved proteins, glucose,
ions, hormones, and gases.
pulmonary circulation The part of the cardiovascular system which carries oxygen-poor blood away from the
heart to the lungs, and returns oxygen-rich blood back to the heart.
systemic circulation The portion of the cardiovascular system which carries oxygen-rich blood away from the
heart to the body, and returns oxygen-poor blood back to the heart.
veins Blood vessels that carry blood back to the heart.
Functions of the Cardiovascular System
Your cardiovascular system has many jobs. It acts as a message delivery service, a pump, a heating system, and
a protector of the body against diseases. Every cell in your body depends on your cardiovascular system. In this
chapter, you will learn how your cardiovascular system works and how it helps to maintain homeostasis.
The cardiovascular system shown in Figure 18.1 is the organ system that is made up of the heart, the blood vessels,
and the blood. It moves nutrients, hormones, gases (such as oxygen) and wastes (such as carbon dioxide) to and
from your cells. It also helps to keep you warm by moving warm blood around your body. To do these tasks, your
cardiovascular system works with other organ systems, such as the respiratory, endocrine, and nervous systems.
The Movement of Gases
The movement of gases, especially oxygen and carbon dioxide, is one of the most important jobs of the cardiovascular system. But the cardiovascular system cannot do this alone. It must work with other organ systems, especially
the respiratory system, to move these gases throughout your body.
Oxygen is needed by every cell in your body. You breathe in oxygen and breathe out carbon dioxide through your
respiratory system. Once oxygen enters your lungs, it must enter your blood stream in order to move around your
body. Oxygen is moved in your blood by attaching to a protein called hemoglobin. The oxygen moves from the
blood into the tissues, while carbon dioxide travels in the opposite direction. Carbon dioxide is transported back to
the lungs, where it moves out of the blood and into your lungs for release from your body.
Parts of the Cardiovascular System
Your heart pushes the blood around your body through the blood vessels. The heart, shown in Figure 18.2 , is made
of cardiac muscle. The heart is connected to many blood vessels that bring blood all around the body. The cardiac
muscle contracts and pumps blood through the blood vessels.
Blood Vessels
The job of the blood vessels is to move the blood around the body. There are three main types of blood vessels in
the body.
a. Arteries are blood vessels that carry blood away from the heart. Arteries have thick walls that have a layer
of smooth muscle, as shown in Figure 18.3 . Arteries usually carry oxygen-rich blood around the body. The
blood that is in arteries is under pressure. The contractions of the heart muscle causes blood to push against
the walls of the arteries. This "push" is referred to as blood pressure. Blood pressure is highest in the arteries
and decreases as the blood moves into smaller blood vessels. Thick walls help prevent arteries from bursting
under the pressure of blood.
b. Veins are blood vessels that carry blood back to the heart. Veins have thinner walls than arteries do, as you can
see in Figure 18.4 . The blood in veins is not under pressure. Veins have valves that stop blood from moving
backward. Blood is moved forward in veins when the skeletal muscles squeeze the veins. Blood that is carried
by veins is usually low in oxygen. The only veins that carry oxygen-rich blood are called the pulmonary veins,
which carry blood to the heart from the lungs.
c. Capillaries these are the tiniest blood vessels in the body. Every cell in the body needs oxygen, but arteries
are too large to bring oxygen and nutrients to single cells. Further from the heart, arteries form capillaries. The
walls of capillaries are only as thick as a single layer of cells. Capillaries connect arteries and veins together,
The cardiovascular system moves nutrients and other substances throughout the body.
Blood is collected in the heart and
pumped out to the lungs where it releases carbon dioxide and picks up oxygen before it is pumped to the rest of the
as shown in Figure 18.5 . Capillaries also send water, oxygen and other substances to body cells, while they
collect carbon dioxide and other wastes from cells and tissues. Capillaries are so narrow that blood cells must
move in single file through them. A capillary bed is the network of capillaries that supply an organ with blood.
The more active a tissue or organ is, the more capillaries it needs to get nutrients and oxygen.
Arteries are thick-walled vessels with
many layers including a layer of smooth
The walls of veins are not as thick as
artery walls veins have valves that stop
blood from flowing backward.
Capillaries connect arteries and veins.
Blood is a body fluid that is a type of connective tissue. Blood is made of blood cells, and a liquid called plasma.
The main types of cells found in blood are red blood cells and white blood cells.
• Red blood cells carry oxygen. Oxygen-rich blood is bright red and oxygen-poor blood is dark red.
• White blood cells fight against infection and disease.
The cardiovascular system of humans is "closed." That means the blood never leaves the blood vessels inside of the
body Other organisms have blood vessels that interact with the environment.
Two Blood Circulation Systems
The blood is pumped around in two large “loops” within the body. One loop moves blood around the body - to the
head, limbs, and internal organs. The other loop moves blood to and from the lungs where carbon dioxide is released
and oxygen is picked up by the blood. A simple version of these two “loops” is shown in Figure 18.6 .
Systemic circulation is the part of the cardiovascular system that carries oxygen-rich blood away from the heart, to
the body, and returns oxygen-poor blood back to the heart. Pulmonary circulation is the part of the cardiovascular
system that carries oxygen-poor blood away from the heart to the lungs, and returns oxygen-rich blood back to the
The Lymphatic System
The lymphatic system is a network of vessels and tissues that carry a clear fluid called lymph. The lymphatic
system, shown in Figure 18.7 spreads all around the body. Lymph vessels are tube-shaped, just like blood vessels.
The lymphatic system works with the cardiovascular system to return body fluids to the blood. The lymphatic system
and the cardiovascular system are often called the body’s two "circulatory systems."
The double circulatory system. Trace
the systemic circulation. Where is the
path of pulmonary circulation
Role of the Lymphatic System in Circulation
You may think that your blood vessels have thick walls without any leaks, but it’s not true! Blood vessels can leak
just like any other pipe. The lymphatic system makes sure leaked blood returns back to the bloodstream.
When a small amount of fluid leaks out from the blood vessels, it collects in the spaces between cells and tissues.
Some of the fluid returns to the cardiovascular system, and the rest is collected by the lymph vessels of the lymphatic
system, which are shown in Figure 18.8 . The fluid that collects in the lymph vessels is called lymph. The lymphatic
system then returns the lymph to the cardiovascular system. Unlike the cardiovascular system, the lymphatic system
is not closed and has no central pump (or heart). Lymph moves slowly in lymph vessels. It is moved along in the
lymph vessels by the squeezing action of smooth muscles and skeletal muscles.
Role of the Lymphatic System in the Body’s Defenses
The lymphatic system also plays an important role in the immune system. The lymphatic system makes white blood
cells that protect the body from diseases.
Organs of the Lymphatic System
Along with the lymph vessels, lymph ducts, and lymph nodes, the lymphatic system also includes many organs. The
tonsils, thymus, and spleen, which are shown in Figure 18.7 , also help prevent diseases. Many of these organs are
also part of the immune system.
If you open your mouth and look at your throat in a mirror, you may see some lumps in the back of your throat.
These are your tonsils. Tonsils are areas of lymphatic tissue on either side of the throat Figure 18.9 . There are also
tonsils in the nasal cavity and behind the tongue. Like other organs of the lymphatic system, the tonsils are also part
of the immune system. The immune system helps protect the body against infection. The tonsils are believed to help
The lymphatic system helps return fluid
that leaks from the blood vessels back
to the cardiovascular system.
Lymph capillaries collect fluid that leaks
out from blood capillaries.
fight off nose and throat, and other upper respiratory tract infections such as colds. Tonsillitis is an infection of the
tonsils that can cause a sore throat and fever.
This image shows the tonsils in the back
of the throat but there are also tonsils in
the nasal cavity and behind the tongue.
Bone Marrow
Bone marrow is the tissue found in the middle of bones. The marrow in the large bones of adults makes new blood
cells, like white blood cells, called T-cells. Other white blood cells, called B-cells, are also created in the bone
The thymus is found in the upper chest. Chemicals made by the thymus help produce cells that fight infection.
White blood cells called lymphocytes move from the bone marrow to the thymus to finish growing. The thymus
grows to its largest size near puberty, and gets smaller as a person ages. If a person’s thymus is surgically removed
or damaged by disease while they are young, the person will be more prone to infection.
The spleen is in the abdomen, as shown in Figure 18.10 . In an area of the spleen called red pulp, materials are
filtered from the blood, including old and dead red blood cells. The spleen also makes red blood cells. Areas called
white pulp help fight infections by making white blood cells. If a person’s spleen is surgically removed, or does
not work properly, the person is at risk for certain infections. You can learn more about the roles of the lymphatic
system and white blood cells in the Diseases and the Body’s Defenses chapter.
FIGURE 18.10
In the spleen the white pulp makes
white blood cells while the red pulp acts
like a filter and removes dead and dying
cells from the blood.
Lesson Summary
• Table 18.1 summarizes the structures and functions of the cardiovascular and lymphatic systems.
TABLE 18.1: Structures and Functions of the Cardiovascular and Lymphatic Systems
Structure (organs and tissues)
Lymph vessels
Lymph nodes
Spleen, tonsils, and adenoids
Blood vessels
Transport fluid (lymph) from between body cells back to blood
Trap invading diseases and cells
with cancer
Trap invading diseases
where white blood cell (lymphocytes) grow larger
Transport blood around the body
Moves oxygen and nutrients; also
carries white blood cells to sites of
infection and inflammation
Pumps blood around the body
• The cardiovascular system includes the heart, the blood vessels, and the blood.
• There are three main types of blood vessels in the body: arteries, veins, and capillaries.
• Systemic circulation is the part of the cardiovascular system that carries oxygen-rich blood away from the
heart, to the body, and returns oxygen-poor blood back to the heart.
• Pulmonary circulation is the part of the cardiovascular system that carries oxygen-poor blood away from the
heart to the lungs, and returns oxygen-rich blood back to the heart.
• Organs of the lymphatic system include the tonsils, thymus, and spleen.
• The lymphatic system works with the cardiovascular system to return body fluids to the blood.
Review Questions
1. Identify the three main parts of the cardiovascular system.
2. Identify three types of blood vessels found in the body.
3. Which blood vessels move blood away from the heart?
4. What are the smallest blood vessels in the body called?
5. Which blood vessels bring blood back to the heart?
6. Identify three main organs of the lymphatic system.
7. Name one function of tonsils.
Apply Concepts
8. Where does blood in the pulmonary system go after it leaves the heart?
9. Where does blood in systemic circulation go after it leaves the heart?
10. What does blood that leaves the heart in systemic circulation have that body cells need?
11. How do the cardiovascular and lymphatic systems work together?
12. What is lymph, and where does it come from?
13. What might happen if a person did not have a spleen?
Critical Thinking
14. Explain how there are actually two circulatory systems in the body.
Further Reading / Supplemental Links
Points to Consider
Next we look further at the heart and blood vessels.
• How can the heart pump blood to the entire body?
• How do you think a hole in the heart muscle affect blood flow?
Heart and Blood Vessels
Lesson Objectives
Describe the structure of the heart.
Outline how blood moves through the heart.
Describe the importance of valves in the heart.
Describe the coronary circulation.
Check Your Understanding
• What is the role of the cardiovascular system?
• What is the main function of the heart?
atrioventricular (AV) valves Valves that stop blood from moving from the ventricles back into the atria.
atrium One of the two small, thin-walled chambers on the top of the heart that blood first enters.
semilunar (SL) valves Found in the arteries leaving the heart; prevents blood flowing back from the arteries into
the ventricles.
valves In the heart; keep the blood flowing in one direction.
ventricles One of the two muscular V-shaped chambers that pump blood out of the heart.
The Heart
What is the heart? How does it pump blood? The heart is divided into four chambers, or spaces: the left and right
atria, and the left and right ventricles. An atrium (singular for atria) is one of the two small, thin-walled chambers
on the top of the heart where the blood first enters. A ventricle is one of the two muscular V-shaped chambers that
pump blood out of the heart. You can remember they are called ventricles because they are shaped like a "V." The
four chambers of the heart are shown in Figure 18.11 .
The atria receive the blood, and the ventricles pump the blood out of the heart. Each of the four chambers of the
heart has a specific job.
• The right atrium receives oxygen-poor blood from the body.
• The right ventricle pumps oxygen-poor blood toward the lungs.
• The left atrium receives oxygen-rich blood from the lungs.
• The left ventricle pumps oxygen-rich blood out of the heart to the rest of the body.
Where is the Heart?
The heart is closer to the center of the body than you may think. It is usually found in the left to middle of the
chest, with the largest part of the heart slightly to the left. It always feels like the heart is on the left side of the body
because the left ventricle is bigger and stronger than the right ventricle. The heart is also surrounded by the lungs.
FIGURE 18.11
The atria receive blood and the ventricles pump blood out of the heart.
Blood Flow Through the Heart
Blood flows through the heart in two separate loops. You can think of them as a “left side loop” and a “right side
loop." The right side of the heart collects oxygen-poor blood from the body and pumps it into the lungs, where it
releases carbon dioxide and picks up oxygen. The left side carries the oxygen-rich blood back from the lungs into
the left side of the heart, which then pumps the oxygen-rich blood to the rest of the body.
The Heartbeat
To move blood through the heart, the cardiac muscle needs to contract in an organized way. Blood first enters the
atria, as shown in Figure 18.12 . When the atria contract, blood is pushed into the ventricles. After the ventricles
fill with blood, they contract, and blood is pushed out of the heart. So how is the blood kept from flowing back on
Valves in the heart keep the blood flowing in one direction. You can see some of the valves in Figure 18.12 . The
valves do this by opening and closing in one direction only. Blood only moves forward through the heart. The valves
stop the blood from flowing backward. There are four valves of the heart.
• The two atrioventricular (AV) valves stop blood from moving from the ventricles to the atria.
• The two semilunar (SL) valves are found in the arteries leaving the heart, and they prevent blood flowing back
from the arteries into the ventricles.
Why does a heart beat? The “lub-dub” sound of the heartbeat is caused by the closing of the AV valves ("lub") and
SL valves ("dub"), after blood has passed through them.
FIGURE 18.12
Blood flows in only one direction in the
heart. Blood enters the atria which contract and push blood into the ventricles.
The atria relax and the ventricles fill
with blood. Finally the ventricles contract and push blood around the body.
Control of the Heartbeat
The heart is a made up of cardiac muscle cells. Cardiac cells are able to contract by themselves. They do not need
help from the nervous system. This is different from skeletal muscle, which needs messages from nerves to contract.
The number of times a heart contracts over a certain amount of time is called the heart rate. Exercising or getting
scared can make the heart rate increase. After the exercise is over, or the fear has passed, the heart rate returns to
Blood Circulation and Blood Vessels
The blood vessels are an important part of the cardiovascular system. They connect the heart to every cell in the
body. Arteries carry blood away from the heart, while veins return blood to the heart. The main arteries and veins of
the heart are shown in Figure 18.13 .
Important Arteries and Veins
There are specific veins and arteries that are more significant than others. The pulmonary arteries carry oxygen-poor
blood away from the heart to the lungs. These are the only arteries that carry oxygen-poor blood. The aorta is the
largest artery in the body. It carries oxygen-rich blood away from the heart. Further away from the heart, the aorta
branches into smaller arteries, which eventually branch into capillaries.
The veins that return oxygen-poor blood to the heart are the superior vena cava and the inferior vena cava. The
pulmonary veins return oxygen-rich blood from the lungs to the heart. The pulmonary veins are the only veins that
carry oxygen-rich blood.
Pulmonary Circulation
Pulmonary circulation is the part of the cardiovascular system that carries oxygen-poor blood away from the heart
and brings it to the lungs. Oxygen-poor blood returns to the heart from the body and leaves the right ventricle
FIGURE 18.13
The right side of the heart pumps deoxygenated blood into pulmonary circulation while the left side pumps oxygenated blood into systemic circulation.
through the pulmonary arteries, which carry the blood to each lung. Once at the lungs, the red blood cells release
carbon dioxide and pick up oxygen when you breathe. The oxygen-rich blood then leaves the lungs through the
pulmonary veins, which return it to the left side of the heart. This completes the pulmonary cycle. The oxygenated
blood is then pumped to the body through systemic circulation, before returning again to pulmonary circulation.
Systemic Circulation
Systemic circulation is the part of the cardiovascular system that carries oxygen-rich blood away from the heart,
to the body, and returns oxygen-poor blood back to the heart. Oxygen-rich blood leaves the left ventricle through
the aorta, then it travels to the body’s organs and tissues. The tissues and organs absorb the oxygen through the
capillaries. Oxygen-poor blood is collected from the tissues and organs by tiny veins, which then flow into bigger
veins, and eventually into the inferior vena cava and superior vena cava. This completes systemic circulation.
The blood releases carbon dioxide and gets more oxygen in pulmonary circulation before returning to systemic
Lesson Summary
• The heart is divided into four chambers, the left and right atria and the left and right ventricles.
• The right side of the heart collects oxygen-poor blood from the body and pumps it into the lungs, where it
releases carbon dioxide and picks up oxygen.
• The left-side carries the oxygen-poor blood back from the lungs into the left side of the heart, which then
pumps the oxygen-poor blood to the rest of the body.
• The valves in the heart prevent blood from flowing backward into the heart.
Further Reading / Supplemental Links
Review Questions
1. Name the four chambers of the heart.
2. Where does oxygen-poor blood first enter the heart?
3. Do ventricles pump blood out of the heart, or do they pump blood into the atria?
4. Do the inferior vena cava and superior vena cava carry oxygen-poor or oxygen-rich blood?
5. To what organ or organs does systemic circulation bring blood?
6. To what organ or organs does pulmonary circulation bring blood?
Apply Concepts
7. Why can the heart be considered to be two separate pumps?
8. What is the purpose of the valves in the heart?
Critical Thinking
9. How might a hole in the heart wall between the two ventricles affect the circulation of blood?
Points to Consider
A more in-depth look at blood is next.
• How do different parts of the blood impact the cardiovascular system?
• How can diet affect how blood carries oxygen?
Lesson Objectives
List the main parts of blood.
Identify three functions of blood.
Name the oxygen-carrying protein found in red blood cells.
Identify the main function of white blood cells.
Describe the importance of the ABO blood system.
Identify three blood disorders or diseases.
Check Your Understanding
• What is the main function of the blood?
• What is the role of oxygen in aerobic (cellular) respiration?
ABO blood type system Blood group system that is determined by the presence or absence of certain molecules,
called antigens, on the surface of red blood cells (RBCs); there are four blood types in the ABO system: A, B,
AB, and O.
antibody Protein that identifies pathogens or other substances as being harmful; can destroy pathogens by attaching to the cell membrane of the pathogen.
blood clotting The complex process by which blood forms solid clots.
fibrin A tough protein that forms strands during the blood clotting process.
hemoglobin Protein that moves oxygen throughout the blood.
hemophilia A group of hereditary diseases that affect the body’s ability to control blood clotting.
leukemia Cancer of the blood or bone marrow; characterized by an abnormal production of blood cells, usually
white blood cells.
lymphoma Cancer of white blood cells called lymphocytes.
platelets Fragments of larger cells that are important in blood clotting.
red blood cells Flattened disk-shaped cells that carry oxygen, the most common blood cell in the blood. Mature
red blood cells do not have a nucleus.
sickle cell disease A blood disease that is caused by abnormally-shaped blood protein hemoglobin.
universal donor A person with type O positive blood; type O red blood cells do not have any antigens on their
membranes and so would not cause an immune reaction in the body of a recipient.
universal recipient A person with type AB positive blood; the blood plasma of AB blood does not contain any
anti-A or anti-B antibodies. People with type AB blood can receive any ABO blood type.
white blood cells Nucleated blood cells that are usually larger than red blood cells; defend the body against infection by bacteria, viruses, and other pathogens.
Components of Blood
Did you know that blood is a tissue? Blood is a fluid connective tissue that is made up of red blood cells, white blood
cells, platelets, and plasma. The cells that make up blood are shown in Figure 18.14 . The different parts of blood
have different roles.
Some of the roles of blood include:
The defense of the body against diseases.
The movement of chemical messages, such as hormones and hormone-like substances.
The control of body temperature.
The repair of damage to body tissues.
If you were to filter out all the cells in blood, plasma is what would be left over. Plasma is the golden-yellow liquid
part of the blood. Plasma is about 90 percent water and about 10 percent dissolved proteins, glucose, ions, hormones,
and gases. Blood is made up mostly of plasma.
Red Blood Cells
Red blood cells (RBCs) are flattened, disk-shaped cells that carry oxygen. They are the most common blood cell in
the blood. There are about 4 to 6 million RBCs per cubic millimeter of blood. Each RBC has 200 million molecules
of hemoglobin. Hemoglobin is the protein that carries oxygen. Hemoglobin also gives the RBCs their red color.
Red blood cells are made in the red marrow of long bones, ribs, skull, and vertebrae. Each red blood cell lives for
only 120 days (about four months). After this time, they are destroyed in the liver and spleen. Red blood cells are
shown in Figure 18.15 . Mature RBCs do not have a nucleus or other organelles.
White Blood Cells
White blood cells (WBCs) are usually larger than red blood cells. They have a nucleus but do not have hemoglobin.
White blood cells make up less than one percent of the blood’s volume. Most WBCs are made in the bone marrow,
and some mature in the lymphatic system. WBCs defend the body against infection by bacteria, viruses, and other
diseases. There are different WBCs with different jobs.
18.3. BLOOD
FIGURE 18.14
A scanning electron microscope SEM
image of human blood cells. Red blood
cells are the flat bowl-shaped cells the
tiny disc-shaped pieces are platelets
and white blood cells are the round cells
shown in the center.
FIGURE 18.15
The flattened shape of RBCs helps
them to carry more oxygen than if they
were rounded.
• Neutrophils can squeeze through capillary walls and swallow particles such as bacteria and parasites.
• Macrophages can also swallow and destroy old and dying cells, bacteria, or viruses. In Figure 18.16 a
macrophage is attacking and swallowing two particles, possibly diseases. Macrophages also release chemical messages that cause the number of WBCs to increase.
• Lymphocytes fight infections caused by viruses and bacteria. Some lymphocytes attack and kill cancer cells.
Lymphocytes called B-cells make antibodies. An antibody is a protein that finds harmful antigens and destroys them. To learn more about the role of WBCs in protecting the body from infection, see the Diseases
and the Body’s Defenses chapter.
FIGURE 18.16
A type of WBC called a macrophage is
attacking a cancer cell
Platelets are very small, but they are very important in blood clotting. Platelets are not cells. They are sticky little
pieces of larger cells. Platelets bud off large cells that stay in the bone marrow. A platelet sits between a RBC and
a WBC in Figure 18.17 . When a blood vessel gets cut, platelets stick to the injured areas. They release chemicals
called clotting factors, which cause proteins to form over the wound. This web of proteins catches RBCs and forms
a clot. This clot stops more blood from leaving the body through the cut blood vessel. The clot also stops bacteria
from entering the body. Platelets survive in the blood for 10 days before they are removed by the liver and spleen.
Transport of Chemical Messages
The blood also acts as a messenge delivery service, like a post office. Chemical messengers called hormones are
carried and brought to cells by the blood.
18.3. BLOOD
FIGURE 18.17
A platelet lies between a RBC at left
and a WBC at right. Platelets are little
pieces of larger cells that are found in
the bone marrow.
Control of Body Temperature
How do your blood vessels control your body temperature? Your blood also moves heat around your body. When
your brain senses that your body temperature is increasing, it sends messages to the blood vessels in the skin to
increase in diameter. Increasing the diameter of the blood vessels increases the amount of blood and heat that moves
near the skin surface. The heat is then released from the skin. What do you think your blood vessels do when your
body temperature is decreasing?
Blood Clotting
Blood clotting is a complex process by which blood forms solid clots. As discussed above, clotting is important to
stop bleeding and begin the repair of damaged blood vessels. Blood clotting disorders can lead to an increased risk
of bleeding or clotting inside a blood vessel. Platelets are important for the proper clotting of blood.
Clotting is started almost immediately when an injury damages the inside lining of a blood vessel. The steps involved
in clotting are described below:
Platelets clump together, forming a clot at the injury site.
Proteins in the plasma cause a chemical reaction that brings a protein called fibrin to the site.
The fibrin forms a web across the platelet clot, trapping red blood cells.
This mass of platelets, fibrin, and red blood cells forms a clot that turns into a scab.
Certain nutrients are needed for the clotting system to work properly. Two of these are calcium and vitamin K.
Bacteria that live in your intestines make enough vitamin K, so you do not need to eat extra vitamin K in your food.
Blood Types
Do you know what your blood type is? Maybe you have heard someone say that they some Type A or Type O blood.
Blood type is a way to describe the type of antigens, or proteins, on the surface of red blood cells (RBCs). There are
four blood types; A, B, AB, and O.
Type A blood has type A antigens on the RBCs in the blood.
Type AB blood has A and B antigens on the RBCs.
Type B has B antigens on the RBCs.
Type O does not have any antigens.
The ABO blood group system is important if a person needs a blood transfusion. A blood transfusion is the process
of putting blood or blood products from one person into the circulatory system of another person. Blood also has
different types of antibodies, or proteins released by the blood cells that attack other strange substances or diseases
in the body. Different blood types have different antibodies (see chart below). What type of antibodies do people
with Type O blood produce? Anti-A and anti-B antibodies. This means that if a person with type O blood received
type A blood, the anti-A antibodies in the person’s blood would attack the A antigens on the RBCs in the donor
blood, as shown in Figure 18.18 . The antibodies would cause the RBCs to clump together, and the clumps could
block a blood vessel. This clumping of blood cells could cause death.
FIGURE 18.18
A person with type O blood has A and B
antibodies in their plasma if the person
was to get type A blood instead of type
O Their A antibodies would attach to the
antigens on the RBCs and cause them
to clump together.
The Rhesus System
The second most important blood group system in human blood is the Rhesus (Rh) system. A person either has, or
does not have, the Rh antigen on the surface of their RBCs. If they do have it, then the person is positive. If the
person does not have the antigen, they are considered negative.
Blood Donors
Recall that people with type O blood do not have any antigens on their RBCs. As a result, type O blood can be given
to people with blood types A, B, or AB. If there are no antigens on the RBCs, there cannot be an antibody reaction
in the blood. People with type O blood are often called universal donors.
The blood plasma of AB blood does not contain any anti-A or anti-B antibodies. People with type AB blood can
receive any ABO blood type. People with type AB positive blood are called universal recipients because they can
receive any blood type. The antigens and antibodies that define blood type are listed in Table 18.2 .
18.3. BLOOD
TABLE 18.2: Blood Types, Antigens, and Antibodies
Blood type
Antigen type
Plasma antibodies
A and B
anti-A, anti-B
Can receive blood
from types
AB, A, B, O
Can donate blood
to types
AB, A, B, O
Blood Diseases
Problems can occur with red blood cells, white blood cells, platelets, and other parts of the blood. Many blood
disorders are genetic, meaning they are inherited from a parent. Some blood diseases are caused by not getting
enough of a certain nutrient, while others are cancers of the blood.
Sickle-Cell Disease
Sickle-cell disease is a blood disease that is caused by abnormally-shaped blood protein hemoglobin. Many of the
RBCs of a person with sickle cell disease are long and curved (sickle-shaped), as shown in Figure 18.19 . The long,
sickle-shaped RBCs can have damaged cell membranes, which can cause them to burst. The long shape of the cells
can cause them to get stuck in narrow blood vessels. This clotting means that oxygen cannot reach the cells. People
with sickle-cell disease are most often well, but can occasionally have painful attacks. The disease is not curable,
but can be treated with medicines.
There is an advantage, however, to sickle-cell disease. People who are carriers for the sickle cell gene, or who are
heterozygous, are resistant to severe malaria. See the Genetics chapter for further information.
FIGURE 18.19
The RBCs of a person with sicklecell disease le f t are long and pointed
rather than straight like normal cells
right . The abnormal cells cannot carry
oxygen properly and can get stuck in
Anemia is a disease that occurs when there is not enough hemoglobin in the blood to carry oxygen to body cells.
Hemoglobin normally carries oxygen from the lungs to the tissues. Anemia leads to a lack of oxygen in organs.
Anemia is usually caused by one of the following:
• A loss of blood from a bleeding wound or a slow leak of blood.
• The destruction of RBCs.
• A lack of RBC production.
Anemia may not have any symptoms. Some people with anemia feel weak or tired in general or during exercise.
They also may have poor concentration. People with more severe anemia often get short of breath during activity.
Iron-deficiency anemia is the most common type of anemia. It occurs when the body does not receive enough iron.
Since there is not enough iron, hemoglobin, which contains iron, cannot be made.
In the United States, 20 percent of all women of childbearing age have iron deficiency anemia, compared with only
2 percent of adult men. The most common cause of iron deficiency anemia in young women is blood lost during
menstruation. Iron deficiency anemia can be avoided by getting the recommended amount of iron in one’s diet.
Anemia is often treated or prevented by taking iron supplements.
Boys and girls between the ages of 9 and 13 should get 9 mg of iron every day. Girls between the ages of 14 and
18 should get 15 mg of iron every day. Boys between the ages of 14 and 18 should get 11 mg of iron every day.
Pregnant women need the most iron — 27 mg daily.
Good sources of iron include shellfish, such as clams and oysters. Red meats, such as beef, are also a good source of
iron. Non-animal sources of iron include seeds, nuts, and legumes. Breakfast cereals often have iron added to them
in a process called fortification. Some good sources of iron are listed in Table 18.3 . Eating vitamin C along with
iron-containing food increases the amount of iron that the body can absorb.
TABLE 18.3: Sources of Iron
Canned clams, drained, 3 oz
Fortified dry cereals, about 1 oz
Roasted pumpkin and squash seeds, 1 oz
Cooked lentils, 12 cup
Cooked fresh spinach, 12 cup
Cooked ground beef, 3 oz
Cooked sirloin beef, 3 oz
Milligrams (mg) of Iron
1.8 to 21.1
Leukemia is a cancer of the blood or bone marrow. It is characterized by an abnormal production of blood cells,
usually white blood cells. Lymphoma is a type of cancer in white blood cells called lymphocytes. There are many
types of lymphoma.
Hemophilia is the name of a group of sex-linked hereditary diseases that affect the body’s ability to control blood
clotting. Hemophilia is caused by a lack of clotting factors in the blood. Since people with hemophilia cannot
produce clots, any cut can put a person at risk of bleeding to death. The risk of internal bleeding is also increased in
hemophilia, especially into muscles and joints.
Lesson Summary
• Blood is a fluid connective tissue that contains red blood cells, white blood cells, platelets, and plasma.
18.3. BLOOD
• The red blood cells give blood its red color.
• Blood carries oxygen and nutrients to body cells and carries wastes away. It also helps to maintain body
temperature and to carry chemical messengers called hormones around the body.
• Hemoglobin is the oxygen-carrying protein that is found in red blood cells.
• White blood cells defend the body against infection by bacteria, viruses and other pathogens.
• Blood type is determined by the presence or absence of certain molecules, called antigens, on the surface of
red blood cells (RBCs).
• There are four blood types; A, B, AB, and O.
• If a person receives the wrong blood type, antibodies in the recipient’s blood will attack the antigens on the
RBCs in the donor blood.
• Sickle-cell disease is a blood disease that is caused by abnormally-shaped hemoglobin, and important blood
• Anemia is a disorder caused by a lack of hemoglobin in the blood.
Review Questions
1. What types of cells are found in blood?
2. What is the liquid part of blood called?
3. What is the function of platelets?
4. Identify one other function of blood other than bringing oxygen to body cells.
5. What is the oxygen-carrying protein found in red blood cells?
6. Identify two ways that white blood cells defend the body from infection.
7. Identify three blood disorders or diseases.
8. Identify two good sources of iron in the diet.
Apply Concepts
9. How are the red blood cells of the different blood groups different?
10. Why are people with type O blood called "universal donors?"
11. Why are people with type AB blood called "universal recipients?"
12. What is a common cause of anemia in young people?
Critical Thinking
13. How can the shape of the hemoglobin protein in a person with sickle-cell disease affect other body systems?
Further Reading / Supplemental Links
Points to Consider
The health of the cardiovascular system is next.
• Why do you think the blood in veins not under pressure?
• How might your diet affect your cardiovascular system?
18.3. BLOOD
Health of the Cardiovascular System
Lesson Objectives
Outline the cause of blood pressure in arteries.
Identify the healthy range for blood pressure.
Describe three types of cardiovascular disease.
Identify things you can do to avoid cardiovascular disease.
Check Your Understanding
• What is the role of the cardiovascular system?
angina Chest pain caused by the lack of oxygen to the heart muscle; can happen during times of stress or physical
atherosclerosis A chronic inflammation of the walls of arteries that causes swelling and a buildup of material
called plaque.
blood pressure The force exerted by circulating blood on the walls of blood vessels.
cardiovascular disease (CVD) Any disease that affects the cardiovascular system, although the term is usually
used to describe diseases that are linked to atherosclerosis.
coronary heart disease The end result of the buildup of plaques within the walls of the coronary arteries.
heart attack Event that occurs when the blood supply to a part of the heart is blocked.
hypertension Also called high blood pressure; a condition in which a person’s blood pressure is always high; the
systolic blood pressure is always 140 mm Hg or higher, and/or their diastolic blood pressure is always 90 mm
Hg or higher.
plaque Cell pieces made up of fatty substances, calcium, and connective tissue that build up around the area of
inflammation; builds up on the lining of blood vessels.
stroke A loss of brain function due to a blockage of the blood supply to the brain.
Blood Vessels and Blood Pressure
The health of your whole body depends on the good health of your cardiovascular system. Blood pressure occurs
when circulating blood puts pressure on the walls of blood vessels. The pressure causes the walls of the arteries to
move in a rhythmic fashion.
Blood in arteries is under the greatest amount of pressure. A person’s pulse is the throbbing of their arteries that
results from the heart beat. The pressure of the circulating blood slowly decreases as blood moves from the arteries,
and into the smaller blood vessels. Blood in veins is not under pressure.
The systolic pressure is the highest pressure in the arteries. The diastolic pressure is the lowest pressure. Pressure in
arteries is most commonly measured by an instrument called a sphygmomanometer, shown in Figure 18.20 . The
height of the column of mercury shows the pressure of the circulating blood. Many modern blood pressure devices
no longer use mercury, but values are still reported in millimeters of mercury (mm Hg).
FIGURE 18.20
A digital sphygmomanometer is made of
an inflatable cuff and a pressure meter
to measure blood pressure.
Healthy Blood Pressure Ranges
Healthy ranges for blood pressure are:
• Systolic: less than 120 mm Hg
• Diastolic: less than 80 mm Hg
Blood pressure is usually written as systolic/diastolic mm Hg. For example, a reading of 120/80 mm Hg is said as
"one twenty over eighty." These measures of blood pressure can change with each heartbeat and over the course of
the day. Age, gender and race also influence blood pressure values. Pressure also varies with exercise, emotions,
sleep, stress, nutrition, drugs, or disease.
Studies have shown that people whose systolic pressure is around 115 mm Hg rather than 120 mm Hg have fewer
health problems. Clinical trials have shown that people who have blood pressures at the low end of these ranges
have much better long term cardiovascular health.
Hypertension, which is also called "high blood pressure," occurs when a person’s blood pressure is always high.
Hypertension is said to be present when a person’s systolic blood pressure is always 140 mm Hg or higher, and/or
their diastolic blood pressure is always 90 mm Hg or higher. Having hypertension increases a person’s chance for
developing heart disease, having a stroke, and other serious cardiovascular diseases. Hypertension often does not
have any symptoms, so a person may not know they have high blood pressure. For this reason, hypertension is often
called the silent killer. Treatments for hypertension include diet changes, exercise, and medication.
Atherosclerosis and Other Cardiovascular Diseases
A cardiovascular disease (CVD) is any disease that affects the cardiovascular system. But the term is usually used
to describe diseases that are linked to atherosclerosis.
Atherosclerosis is an inflammation of the walls of arteries that causes swelling and a buildup of material called
plaque. Plaque is made of cell pieces, fatty substances, calcium, and connective tissue that builds up around the area
of inflammation. As a plaque grows, it stiffens and narrows the artery, which decreases the flow of blood through
the artery, shown in Figure 18.21 .
FIGURE 18.21
Atherosclerosis is sometimes referred
to as hardening of the arteries plaque
build-up decreases the blood flow
through the artery.
Atherosclerosis normally begins in later childhood, and is usually found in most major arteries. It does not usually
have any early symptoms. Causes of atherosclerosis include a high-fat diet, high cholesterol, smoking, obesity, and
diabetes. Atherosclerosis becomes a threat to health when the plaque buildup prevents blood circulation in the heart
or the brain. A blocked blood vessel in the heart can cause a heart attack. Blockage of the circulation in the brain
can cause a stroke. According to the American Heart Association, atherosclerosis is a leading cause of CVD.
Coronary Heart Disease
Hearts have arteries that require oxygen, too. Muscle cells in the heart are given oxygen by coronary arteries.
Blocked flow in a coronary artery can result in a lack of oxygen and the death of heart muscle. Coronary heart
disease is the end result of the buildup of plaques within the walls of the coronary arteries.
Coronary heart disease often does not have any symptoms. A symptom of coronary heart disease is chest pain.
Occasional chest pain, called angina can happen during times of stress or physical activity. The pain of angina
means the heart muscle fibers need more oxygen than they are getting. Most people with coronary heart disease
often have no symptoms for many years until they have a heart attack.
A heart attack happens when the blood cannot reach the heart because a blood vessel is blocked. If cardiac muscle
is starved of oxygen for more than about five minutes, it will die. Cardiac muscle cells cannot be replaced, so once
they die, they are dead forever. Coronary heart disease is the leading cause of death of adults in the United States.
How a blocked coronary artery can cause a heart attack, and cause part of the heart muscle to die, is shown in Figure
18.22 . If part of the cardiac muscle becomes injured, the heart will not work as well as it used to.
FIGURE 18.22
A blockage in a coronary artery stops
oxygen getting to part of the heart muscle so areas of the heart that depend on
the blood flow from the blocked artery
are starved of oxygen.
Atherosclerosis in the arteries of the brain can also lead to a stroke. A stroke is a loss of brain function due to a
blockage of the blood supply to the brain. It can be caused by a blood clot, an object that gets caught in a blood
vessel, or by a bleeding blood vessel. Risk factors for stroke include old age, high blood pressure, having a previous
stroke, diabetes, high cholesterol, and smoking. The best way to reduce the risk of stroke is to have low blood
pressure. Many other risk factors, however, such as avoiding or quitting smoking are also important.
Keeping Your Cardiovascular System Healthy
There are many risk factors that can cause a person to develop CVD. A risk factor is anything that is linked to an
increased chance of developing a disease or an infection. Some of the risk factors for CVD you cannot control, but
there are many risk factors you can control.
Risk factors you cannot control include:
• Age: the older a person is, the greater their chance of developing a cardiovascular disease.
• Gender: men under age 64 are much more likely to die of coronary heart disease than women, although the
gender difference decreases with age.
• Genetics: family history of cardiovascular disease increases a person’s chance of developing heart disease.
Risk factors you can control include:
• Tobacco smoking: giving up smoking or never starting to smoke is the best way to reduce the risk of heart
• Diabetes: diabetes can cause bodily changes, such as high cholesterol levels, which are are risk factors for
• High cholesterol levels: high amounts of low-density lipids in the blood, also called "bad cholesterol," increase the risk of CVD.
• Obesity: being obese, especially if the fat is mostly found in the upper body, rather than the hips and thighs,
increases risk significantly.
• High blood pressure: hypertension can cause atherosclerosis.
• Lack of physical activity: aerobic activities, such as the one shown in Figure 18.23 , help keep your heart
healthy. To reduce the risk of disease, you should be active for at least 60 minutes a day, five days a week.
• Poor eating habits: eating mostly foods that do not have many nutrients other than fat or carbohydrate leads
to high cholesterol levels, obesity and CVD (Figure 18.24 ).
FIGURE 18.23
Sixty minutes a day of vigorous aerobic
activity such as basketball is enough
to help keep your cardiovascular system
Lesson Summary
Blood pressure is the force put on the walls of blood vessels by circulating blood.
The force put on the walls of arteries is called blood pressure.
Blood pressure is measured by an instrument called a sphygmomanometer.
In the United States, the healthy ranges for systolic pressure is less than 120 mm Hg and a diastolic pressure
of less than 80 mm Hg.
FIGURE 18.24
The USDA&#8217 s MyPyramid recommends that you limit the amount of such
foods in your diet to occasional treats.
• Hypertension occurs when a person’s blood pressure is always high.
• A cardiovascular disease (CVD) is any disease that affects the cardiovascular system. Atherosclerosis, coronary heart disease, and stroke are examples of CVDs.
• Cardiovascular diseases are lifestyle diseases. Having a poor diet and not getting enough exercise are two
major causes of CVD.
Review Questions
1. What is the cause of blood pressure?
2. What is the healthy range for blood pressure?
3. When is a person considered to have hypertension?
4. What is atherosclerosis?
5. What are three risks factor for cardiovascular disease?
Apply Concepts
6. How is the pulse related to blood pressure?
7. Is the blood in veins under pressure? Explain your answer.
8. Why is hypertension called a silent killer?
9. A stroke could be thought of as a "brain attack," in a similar way to a heart attack. How are strokes and heart
attacks similar?
10. What is the difference between a controllable risk factor and an uncontrollable risk factor?
11. Why are cardiovascular diseases called lifestyle diseases?
Critical Thinking
12. One of your friends says, "Heart disease is genetic and people have it in my family, so there is nothing I can do
about it." Explain to your friend how cardiovascular disease can be a lifestyle choice. Also, give your friend three
recommendations on things he/she can do to prevent cardiovascular disease.
Further Reading / Supplemental Links
Points to Consider
Next we take a look at the respiratory system.
• Do you think there is a relationship between the cardiovascular system and the respiratory system? What
could it be?
• Do you think hypertension affects the ability of the blood to release carbon dioxide and pick up oxygen in the
lungs? Why?
19MS Respiratory and Excretory
The above image shows the tissue found inside of the lungs. The lungs contain alveoli. Alveoli absorb oxygen and
send it to the blood vessels. They also move carbon dioxide from the blood vessels back to the lungs to be exhaled.
Alveoli look like clumps of grapes. Why do you think it is important that your body have many alveoli? What would
happen if your alveoli lost their ability to function? Why do you think alveoli are shaped like spheres?
The respiratory system is important because it brings oxygen to cells in your body. But it also removes a waste,
carbon dioxide. Another system in your body, the excretory system, also removes wastes. The excretory system
moves waste from your digestive system and from your blood out of your body.
How do the respiratory system and excretory system work together? How does damage in one system affect the
other? Consider these questions about respiration and waste removal as you read the following chapter.
The Respiratory System
Lesson Objectives
Identify the parts of the respiratory system.
Identify the main function of the respiratory system.
Describe how breathing works.
Outline how the respiratory system and the cardiovascular system work together.
Identify how breathing and cellular respiration are connected.
Check Your Understanding
• What is an organ system?
• What is the role of the circulatory system?
• How does your blood get oxygen?
alveoli Little sacs at the end of the bronchioles where most of the gas exchange occurs.
diaphragm A sheet of muscle that extends across the bottom of the rib cage.
epiglottis A flap of connective tissue that closes over the trachea when food is swallowed; prevents choking or
inhaling food.
exhalation Pushing air out of the body through the nose or mouth.
external respiration The movement of oxygen into the body and carbon dioxide out of the body.
gas exchange The movement of oxygen across a membrane and into the blood and the movement of carbon dioxide out of the blood.
inhalation Taking air into the body through the nose and mouth.
internal respiration The exchange of gases between the blood and the cells of the body.
larynx Found just below the point at which the pharynx splits into the trachea and the esophagus. Your voice
comes from your larynx; air from the lungs passes across thin membranes in the larynx and produces sound;
also called the voicebox.
pharynx A long tube that is shared with the digestive system; both food and air pass through the pharynx.
respiration The process of getting oxygen into the body and releasing carbon dioxide.
respiratory system The organ system that allows oxygen to enter the body and carbon dioxide to leave your body.
trachea A long tube that leads down to the chest where it divides into the right and left bronchi in the lungs; also
called the windpipe.
Roles of the Respiratory System
You breathe mostly without thinking about it. Remember how uncomfortable you felt the last time you had a cold
or a cough? You usually do not think about your respiratory system or how it works until there is a problem with it.
Every cell in your body depends on your respiratory system.
Your respiratory system is made up of the tissues and organs that allow oxygen to enter the body and carbon dioxide
to leave your body. Organs in your respiratory system include your:
These structures are shown in Figure 19.1 .
Parts of the Respiratory System
Figure 19.1 shows many of the structures of the respiratory system. Each of the parts has a specific job. The parts
of the respiratory system include the following:
• The diaphragm is a sheet of muscle that spreads across the bottom of the rib cage. When the diaphragm
contracts, the chest volume gets larger and the lungs take in air. When the diaphragm relaxes, the chest
volume gets smaller and air is pushed out of the lungs.
• The nose and nasal cavity filter, warm, and moisten the air you breathe. The nose hairs and mucus produced
by the cells in the nose catch particles in the air and keep them from entering the lungs. When particles in the
air do reach the lungs, what do you think happens?
• Behind the nasal cavity, air passes through the pharynx, a long tube. Both food and air pass through the
• The larynx, also called the "voice box," is found just below the pharynx. Your voice comes from your larynx.
Air from the lungs passes across thin tissues in the larynx and produces sound.
• The trachea, or windpipe, is a long tube that leads down to the lungs, where it divides into the right and left
bronchi. The bronchi branch out into smaller bronchioles in each lung.
• Since food goes down the pharynx, how is it stopped from entering the trachea? A flap of tissue called the
epiglottis closes over the trachea when food is swallowed to prevent choking or inhaling food.
The respiratory system. Air moves in
through the nose and mouth and down
the trachea which is a long straight tube
in the chest.
• The bronchioles lead to the alveoli. Alveoli are the little sacs at the end of the bronchioles. They look like
little bunches of grapes, as shown in Figure 19.2 . Oxygen is exchanged for carbon dioxide in the alveoli.
Gas exchange is the name we give to the process that allows oxygen to enter the blood and carbon dioxide to
move out of the blood - the two gases are "exchanged."
The alveoli are the tiny grape-like structures in the lungs and the sites of gas
How We Breathe
Most of the time, you breathe without thinking about it. Breathing is mostly an involuntary action that is controlled
by a part of your brain that also controls your heart beat. If you swim, do yoga, or sing, you know you can also
control your breathing. Taking air into the body through the nose and mouth is called inhalation. Pushing air out
of the body through the nose or mouth is called exhalation. The man in Figure 19.3 is exhaling before he surfaces
from the pool water.
How do lungs allow air in? As mentioned above, air moves into and out of the lungs by the movement of muscles.
The diaphragm and rib muscles contract and relax to move air into and out of the lungs. During inhalation, the
diaphragm contracts and moves downward. The rib muscles contract and cause the ribs to move outward. This
causes the chest volume to increase. Because the chest volume is larger, the air pressure inside the lungs is lower
than the air pressure outside. This difference in air pressures causes air to be sucked into the lungs. When the
diaphragm and rib muscles relax, air is pushed out of the lungs. Exhalation is similar to letting the air out of a
The walls of the alveoli are very thin and allow gases to enter into them. The alveoli are lined with capillaries. These
capillaries are shown in Figure 19.4 . Oxygen moves from the alveoli to the blood in the capillaries that surround
the alveoli. At the same time, carbon dioxide moves in the opposite direction, from capillary blood to the alveoli.
Being able to control breathing is important for many activities such as swimming. The man in the photograph is exhaling before he surfaces from the water.
The bronchi and alveoli. During respiration oxygen gets pulled into the lungs
and enters the blood by passing across
the thin alveoli membranes and into the
Breathing and Respiration
When you breath in, oxygen is drawn in through the mouth and down into the lungs. The oxygen then passes across
the thin lining of the capillaries and into the blood. The oxygen molecules are carried to the body cells by the blood.
Carbon dioxide from the body cells is carried by the blood to the lungs where it is released into the air. The process
of getting oxygen into the body and releasing carbon dioxide is called respiration.
Sometimes breathing is called respiration, but there is much more to respiration than just breathing. There are
actually two parts to respiration, external respiration and internal respiration. External respiration is the movement
of oxygen into the body and carbon dioxide out of the body. Internal respiration is the exchange of oxygen and
carbon dioxide between the blood and the cells of the body (Figure 19.5 ).
The Journey of a Breath of Air
Breathing is only part of the process of bringing oxygen to where it is needed in the body. After oxygen enters the
lungs, what happens?
The oxygen enters the blood stream from the alveoli. Then, the oxygen-rich blood returns to the heart.
Oxygen-rich blood is then pumped through the aorta.
From the aorta, oxygen-rich blood travels to the smaller arteries and finally to the capillaries.
The oxygen molecules move out of the capillaries and into the body cells.
While oxygen moves from the capillaries and into body cells, carbon dioxide moves from the cells into the
Gas exchange is the movement of oxygen into the blood and carbon dioxide
out of the blood.
Breathing and Cellular Respiration
The oxygen that arrives at the cells from the lungs is used by the cells to help release the energy stored in molecules of
sugar. Cellular respiration is the process of breaking down glucose to release energy (see the Cell Functions chapter).
The waste products of cellular respiration include carbon dioxide and water. The carbon dioxide molecules move
out of the cells and into the capillaries that surround the cells. As explained above, the carbon dioxide is removed
from the body by the lungs.
Lesson Summary
• Your respiratory system is made up of the tissues and organs that allow oxygen to enter and carbon dioxide to
leave your body.
• Respiratory system organs include your nose, mouth, larynx, pharynx, lungs, and diaphragm.
• During inhalation, the diaphragm contracts and moves downward, and brings air into the lungs.
During exhalation, the diaphragm and rib muscles relax and air is pushed out of the lungs.
• Oxygen enters the lungs, then passes through the alveoli and into the blood. The oxygen is carried around the
body in blood vessels.
• Carbon dioxide, a waste gas, moves into the blood capillaries and is brought to the lungs to be released into
the air during exhalation.
• The oxygen that arrives from the lungs is used by the cells during cellular respiration to release the energy
stored in molecules of sugar.
Review Questions
1. Name the five main organs in the respiratory system.
2. What is the main function of the respiratory system?
3. In what part of the lung does gas exchange occur?
4. What is the important gas that is carried into the lungs during inhalation?
Apply Concepts
5. A classmate says that lung muscles cause the lungs to move during breathing. Do you agree with your classmate?
6. How do the respiratory system and the cardiovascular system work together?
7. Breathing is an involuntary action. Does this mean that you cannot control your breathing?
8. What is the difference between breathing and respiration?
9. What is the name of the waste gas that is released during exhalation?
Critical Thinking
10. If a disease caused the alveoli to collapse, how might this affect a person’s health?
Further Reading / Supplemental Links
Points to Consider
• How do you think the health of your respiratory system might affect the health of other body systems?
Health of the Respiratory System
Lesson Objectives
Identify the organs affected by a respiratory disease.
Identify how a respiratory disease can affect the rest of the body.
Describe how asthma affects breathing.
Outline how smoking affects the respiratory system.
Identify what you can do to keep your respiratory system healthy.
Check Your Understanding
• What is the role of the respiratory system?
• What are some of the organs in the respiratory system?
allergen A substance that triggers an allergy.
asthma A chronic illness in which the bronchioles are inflamed and become narrow.
bronchitis An inflammation of the bronchi.
chronic disease A disease that lasts for a long time, perhaps a few years or longer.
chronic obstructive pulmo